You're right. Migrating playlist from one platform, or just a player, to another is huge problem. Keeping them in sync seems impossible.

A universal cross-platform playlist format seems like a much needed feature, but I don't se ehow this could work without significant changes in every supported player/software.

Here's a very real playlist issue I'm facing right now: I use Roon as my main audio player / music management solution. Roon manages my library which is located on a NAS. I have a few playlists created there, but no way to sync them to my iPhone unless I manage a separate playlist in iTunes which has access to the very same NAS library. It's not a deal breaker since I'm used to playlists being software-specific.

Unfortunately I don't see that changing anytime since everyone's answer to this problem seems to be using one single streaming service - a solution which, frankly, handles this particular problem very well, but doesn't work for me due to disadvantages of streaming discussed earlier in this thread.

The solution for that is what I do now: You keep one canonical copy of your playlists in UPL and use that to export to PLS or copy files to a directory, as needed. Then you import that playlist/files to your devices.

It would perhaps need an accompanying software service that could handle integration between music services you use. Almost like a music librarian.