A can light has a removable, standard bulb

The housing that the bulb goes inside looks a bit like a can, thus the name. You've probably seen this kind of bulb before:

A canless light has the actual LED bit built in

This means they can be more creative with their shape, including being almost completely flat and/ or surface mounted. But if the LED(s) fail, you have to replace the whole unit. That could be problematic if the unit isn't being manufactured anymore.

Just to add some of the counterpoints, copying standard E27 lightbulb form factor results in a terrible form factor for an LED fixture. As a result, LEDs can be more efficient in a puck type fixture as well as shed heat better or have a much longer 80% brightness lifespan as the casing is designed to shed heat whereas traditional pot lights are designed to stop an incandescent bulb of a certain wattage from starting a fire, which means they are sometimes insulated and not good at shedding heat. Because the bulbs are designed to produce omnidirectional light and pot lights rarely have good reflectors and are highly directional, LEDs on a flat surface have another big efficiency advantage. You can get the best of both worlds by using commercial fixtures with replaceable COB LED modules and drivers, but they're expensive. The lifetime of good COBs is extremely long and some brands make new generation LEDs as more efficient drop in replacements at the same power level. Good driver modules can last an extremely long time as well.

As a little extra background I'll add a link to one of my answers on EE.SE, but a synopsis of what is relevant to this question:

The figure of relevance is the Luminous efficacy of an LED fixture (Perceived amount of light per unit power) at a given CRI (Quality of color depth)

Luminous efficacy can be improved by any of the following and how a lightbulb shaped form factor is relevant to a gain of 10% or more luminous efficacy:

• Using better LEDs (Lightbulb shape is fine and cheap to update although it's hard to make good comparisons without testing bulbs yourself, this applies to all fixtures.)

• Using more LEDs than are necessary (Lightbulb shape has a disadvantage due to limited space and low cost, and an advantage in high purchase volume).

• Keep the LEDs and drive circuit cooler. Use a larger or better shaped heat sink and avoid insulative layers. (Lightbulb shape and standard can fixtures have disadvantages here, even if not insulated type. The bulb has limited space and poor shape for a heat sink, is surrounded by air, a poor thermal conductor, and hot air moves upwards, so with a downward facing can, hot air doesn't circulate well, giving the bulb a higher ambient.)

• Using high efficiency switched mode constant current control and only using PWM dimming at low currents (Driver quality varies widely in LED lightbulbs, could be anything from a low efficiency rectifier and capacitive dropper with a series resistor to a decently efficient constant current switched mode supply. If you have more space or money available you can build a better driver, so bulbs have a strong disadvantage)

• LEDs are highly directional compared to traditional light sources. Bouncing or lensing light wastes luminous efficacy. Position the LEDs so as to distribute the light with minimal lensing and reflection. (Bulbs by nature need to fill the purpose of the object they're replacing, so they have to waste this advantage of using LEDs by being omnidirectional. You can lose 10% or more efficacy in a single lens or reflection even if you selected it well. The cost of extra high efficiency reflectors or lenses also works against manufacturing cost for the pot light fixtures, although this applies to high end replaceable COB fixtures as well.)

I like lightbulbs too, but I believe it's worth working on maximising the efficiency of LEDs, and that can't currently be done with the bulb form factor, so I spread awareness of the disadvantages of the form factor. If the need for improvement becomes common knowledge, perhaps we'll get a standard form factor appropriate to the technology.

An LED bulb beats a CFL beats an incandescent, but an optimally designed LED fixture beats an LED bulb when efficiency and lifespan are more important than cost. Where I live, if you disregard cost of the pot light fixture, you can buy 8.2 LED bulbs for the price of one recessed puck, which will likely outlast the puck, but it hurts no one to know the disadvantages of the E27 form factor. If you don't disregard fixture cost, you can buy 2 LED pucks for the cost of a can fixture and 8 bulbs, so aside from the potentially longer lifespan, you can afford to buy extra pucks.

The main caveat with this is it's hard to tell which are actually the better designed fixtures or bulbs and the bulbs have a lower cost of experimentation. If you wait until one of the pucks dies, by that time the others may have aged to 70% brightness if they weren't designed for long life, and the replacement could be noticeably brighter. Some of the first, cheapest 60W dimmables I bought (Bulbs were still around 6-8 currency and these suddenly appeared for 2 currency) have been among the longest lasting, brightest and most efficient, so unless you're looking for high CRI, more expensive bulbs are not necessarily better.