How practical are clipless pedal shoes on a long bike tour?

SPD pedal compatible shoes are available in many styles intended for many uses. You should be able to find a pair that works for you. There are shoes that are designed for walking as well as riding and have a recessed cleat.

Get shoes from a quality brand and you will have no problem with robustness of the shoe itself . SPD cleat and pedal are designed for mountain biking are are very reliable.

Many shoes are designed with ventilation and are not waterproof, but you can deal with this with shoe covers.

Feet are not universal and so the answer isn't either.

In particular is the question of hotspots, where concentrated forces on the foot cause discomfort and pain.

Most cyclists will find that the only practical clipless shoes for touring have at least a recessed cleat and typically also will fall on the more walkable, flexible-soled end, as opposed to racier shoes with inflexible soles. Within these constraints, there are some riders who can't get clipless to work for them over long days pedaling regardless of the system they choose.

There are now a number of two-bolt pedals that have some of the broader shoe contact and hotspot avoidance benefits of road pedals, but work with recessed cleats. These are what you should really be looking at if you're buying pedals for touring. The double-sided "trail" type mountain SPDs, PD-M8120 for example, are a commonly encountered example that can work, but the best are the single-sided touring SPDs, like PD-ES600, PD-A600, and PD-A520. (I realize this veers into product recommendation territory; I would recommend looking at this category without naming models if there were more than just a few models to choose from, or more manufacturers than just Shimano doing it.) These sorts of pedals are the main option for attempting to deliver the best of both worlds type setup you want for touring, where you can use a highly walkable shoe but the platform has decent load-spreading ability.

The nature of pedals like this is that fiddling with cleat shims is more likely to be necessary than otherwise, because you're trying to get and keep the engagement and disengagement dialed while also dealing with the broader platform being squashed up against your shoe more than a mountain pedal would be.

I wouldn’t recommend it. The main advantage of bicycling specific shoes is the stiff sole. The cleats which make sure you can’t slip off the pedal are just an additional benefit.

The stiff sole will make walking uncomfortable and the metal cleats are extremely slippery on hard surfaces like cobblestones or stone stairs.

For short distances (like a kilometer or so) you might be okay with softer bicycling shoes. Personally I always brought a second, lightweight pair of shoes (Merrel Vapor Glove, 320g for the full pair and pack down very small) or went barefoot.

I've been known to wear SPD shoes all day, including a few miles of city walking. I can also run the mile from work to the station in mine. There are plenty of mountain/touring shoes worth a look. My preferred ones are no longer made, but my giro rumble are similar, just a little less comfortable.

I've also worn them around camp but on longer trips would take either flip-flops (also known by many other names) or neoprene water shoes, because I like a dip.

I'd say on a long tour the bigger issue is drying them out if they get wet.

Rather than a performance increase, a well-adjusted set f clipless shoes/pedals keeps your feet in the right place and protects your knees. Badly adjusted (including saddle position) they have the opposite effect. You can get some of the benefit with half clips - strapless toe clips and wear whatever you like. They'd be good for riding if you really want hiking boots with you.

Edit to address one more point. The clips on the pedals are seriously robust. SPDs work caked in mud, and will go thousands of km without maintenance. Mine occasionally (as in maybe twice a year, or after 5000 km, but it might be related more to wet storage) need a few drops of oil if clipping out gets stiff, but that's something you could do before you set off

I use MTB clipless shoes for long tours, and would generally recommend it – but also to take other shoes as well, which means it may end up being more sensible to just keep using those only and on flat pedals. But it depends on both you the rider and the terrain of the tour.

This summer I did my first long tour here in Norway, and didn't manage to pedal all the hills with my baggage. So I pushed quite a bit instead, and, using the bike shoes (Giro Terraduro), this gave me blisters. For climbing over rocks they're even worse – on rounded surfaces, the cleats keep touching down and slipping. (I did have a pair with sandals with me also, but next time on a similar tour I'd rather opt for light running shoes.)

On the other hand, when it comes to the pedalling alone I personally wouldn't want to go without the clips – though I use flat pedals for most of my shorter MTB excursions, pedalling on them for longer time with a backpack tends to give me back pains.
Still – generally speaking, in my opinion flat pedals are ok; at least with the sticky MTB ones the power transmission is hardly worse than with clips. (Importantly, they do allow you to use the hamstrings, unlike slippy plastic pedals.) Only for sprints and when going fast over rough terrain (rock gardens, big roots) do clips give significantly better performance, but that's hardly relevant on a tour.

The purpose of clipping one's shoes to the pedal is not to prevent a foot from slipping off the pedal. The purpose is to increase the range of power transmission from one's leg to the cranks.
If you lack toeclips/straps or clipless pedals/shoes, you can only stand on the pedal and apply force for, say 90 degrees of each leg's travel. With your foot attached to the pedal you can increase that to some 300 degrees. On each leg. You can pull up on your rear leg and supplement the force you're applying on the fore pedal.
In fact you can apply more force to the cranks than what you weigh. By a lot. It vastly increases your power. Watch a couple of sprinters accelerate on the track. Cycling shoes have as stiff a sole as possible. My Adidas are completely inflexible. This prevents sore arches. In addition, power transmission isn't wasted in flexing and warming up the sole.
If you're a tourist, carry a pair of walking shoes to put on when you get there. My toeclip/strap shoes, with compulsory cleats, are simply not walkable. Besides, I'd stuff a pebble into the slot and have to pry it out. Modern SPD and their ilk force their adherents to go clipless while not riding in their cycling shoes. That's one of the reasons that those useless platform pedals are so prevalent these days. Toeclips and straps let you use runners for junk riding. And, have the benefits of the clips and straps. Buy a second pair of pedals. One outfitted with SPD and the other with toeclips/straps. Change your pedals to suite the type of riding you're currently engaged in.

Downhill and freeride MTB gear will be your best bet. They are more casual than XC style shoes and FAR better than road shoes in this context.

Shoes:

• Downhill shoes usually have recessed cleat mounting holes.
• DH shoes have soles that are flexible enough to be comfortable walking all day, while stiff enough to provide a decent amount of support while pedaling. I used to ride MTB with flat pedals and running shoes, and I was always fighting the soft foam soles. You will want at least some level of foot support.
• They will be designed to give plenty of off-bike grip. Some have tread similar to that of a hiking boot, which allows MTB riders to build jumps and then ride them immediately afterwards. For you, that will give you much better off-bike walking grip.
• You can get DH shoes with waterproof membranes. This way, you get some level of water resistance without wearing a full on winter boot.
• The construction is usually quite heavy-duty, and they don’t look as flashy/unusual compared to more race-inspired shoes.

Pedals:

• DH clipless pedals have both a clip mechanism and pins on both sides. This way, you can easily ride clipped in, and it will at least be pleasant riding with normal shoes as well. The large contact area also helps mitigate the lack of support you get with a semi-flexible DH shoe sole.
• They’re often built in a more robust way to cope with extreme MTB use.

That said, the fit of the shoe is also very important. If the shoes aren’t right for your feet, you will get pain and hot spots. Make sure to take your new shoes out for a few rides to break them in and make sure they’re the right fit for you before embarking on the big trip.

I've toured with SPD shoes and pedals, but any time that I'm expecting to do more walking than into my accommodation or a shop for food, I would take normal walking shoes/boots as well.

SPD shoes and pedals are very robust and mine have lasted for years with minimal maintenance.

The problem is with "They are nice to walk". They're not. Even in shoes with a recessed cleat and shoe rubber either side, like the Shimano RT82's that I use, the cleat scrapes noisily on the ground and in some cases compromises grip. I can walk in them, but not quickly or comfortably. Perhaps other designs would be better, but they are going to be less flexible and, if the cleat is not covered, sometimes slippery.

There are some benefits of clip pedals, but any performance increase appears to be negligible.

For those reasons, if you're set on taking only one set of shoes, I would start from the other direction.

Find some decent walking shoes which are comfortable for cycling.

I've noticed that walking boots can result in sweaty feet when cycling or rubbing above the ankles from the repeated flexing. Hiking trainers seem to work quite well for me on the bike, although I am careful to tuck the laces in so that they don't get caught on the chainring.

I toured 3000 miles in clipless pedals (not SPDs per se, but using the SPD-compatible shoes), and commuted with SPDs, and currently train with them. Some observations:

• Shimano-branded SPD pedals are high-quality pedals. I wouldn't worry about them breaking down more than any other pedal.
• Some SPD-compatible shoes have relatively walking-friendly soles, some do not. It comes down to how flexible the sole is. I've walked about 3 miles at a stretch in a pair of "walking-friendly" SPDs. They're not awful for that, but they're not foot-pillows either.
• Rigid soles are a benefit if you are putting in long miles every day, as they spread the pressure over more foot area and reduce the chance of hotfoot. Obviously the shoes need to fit well.
• When touring, I carried a pair of very light running shoes. Even on flat pedals, these would have been inappropriate for riding. I was packing light, but these were worth having.