How bad is it to have low spots in a mini-split line set?

Some things you should hire a pro for - which is a generally frowned upon response around here, but in this case it's illegal (In the USA, anyway) to install most mini-split systems without the approriate refrigeration / EPA license for the refrigerant used. Along with that comes the appropriate tooling and experience, normally. Then your system does not leak and release refrigerant (bad for the planet, and also bad for your system.)

In addition, excess line length makes the system inefficient .vs. just enough, even if you arrange the excess so it does not trap refrigerant.

one of the diy systems with precharged line sets there instructions specifically state to coil the excess at the system.

Will a low spot be a real problem? I have not seen one although in theory it sounds like it should. Remember the oil is actually carried with the gas/ liquid to an extent so it is being pushed through the system I doubt you will have problems.

The precharged line sets I have seen are 25’ long and when I install a mini split on a single story I usually use less than 12’ so that would leave 13’ to coil up at the compressor.

Get a flaring tool expensive pro ones are under 30$ Cheap ones under 10$. Cut and re flair. Honestly even if I purchase line sets I almost always cut factory flairs off because they are usually sloppy and uneven. On the systems I have pumped down for others the leaks have always been at the factory flairs.

I reflair and wet the surface with nylog (a sealant made for this purpose) torque them down and pump down again with no issues. The extra time costs so don’t be afraid to flair it is quite easy.

As others have mentioned charging a system requires special licenses and training and equipment in most of the world. When the system is pumped down leaks are usually very obvious but the leak location can be harder to find quality flairs are not hard to make just leave an extra foot at the compressor, practice on the pice you cut off then do the real one. (Don’t forget to put the nut on first I have forgotten this when having a beer and BSing with a friend while doing his system, you figure it out real fast lol).