The biggest issue for me is the lack of consensus of these apps across a longer timeframe. Some apps I used did break or stopped being actively developed (like RiMusic) and then it's another cycle of research and testing until next breakage. Older threads on Reddit are full of suggestions that are no longer working.
The other root issue is because they can break or become abandoned, it's difficult to export/import all playlists, collections and favourites each time.
Say if I use Metrolist now and in 10months it becomes abandoned, I'll have to move but I don't think they offer suitable export format and it's likely that the new app of choice will not allow their format to be imported.
For something like AntennaPod, there is a lot of confidence that they will be around and worst case scenario, I can at least export my podcast list to another app - it may not contain favourites, archived or played podcasts but the main data will be there. Same for Grayjay or Invidious, you can expect them to keep being around and, if needed, to transfer data over without too much hassle.
Please educate me if I am wrong, but this was my experience. I struggle to find reliable and resilient service and app that can allow easy means of data import and export. I'd gladly pay for these benefits. I do keep my Deezer subscription for now because some times you just want to listen to music and not spend time tinkering around settings or forum issue trackers. I also would like to keep my favourites, at least.
Once I find a (private) alternative, I'd gladly transfer the fees to that service, I am still trying to find a reliable, functional alternative that can support required export import formats. Especially if I could recommend the service to my friends.
To add, I spent couple hours last week trying to export Deezer music data (I know, it's Deezer locking in their users) but eventually did it via suggested websites. Tried to import to Metrolist and it keeps crashing, their forum page has similar reports but I don't think it's resolved. Tested another import with 30songs and it worked; tried to edit main file to make it smaller, Metrolist can't accept the edited csf file etc. I may persevere and keep trying to import the favourites list (1k songs) but then I don't see that their export option can be imported to other apps.