Server side's actually okay, I think the android app has a number of bugs. For example I tried to set a folder with several thousand pieces of media and a few other files to auto upload and it'd keep triggering "nextcloud is not responding" pop-ups on my phone. It's ~6 gigs but didn't complete for two painful days. It was almost impossible to open the app cos it's intensely trying to sync that folder.
I thought I managed to cancel it yesterday and remove the folder from auto uploads but today it's trying to upload the contents of that folder again even though I can't find it in my auto upload setup.
I've also felt it misses instant uploads once in a while but never investigated to prove that or the circumstances so take that with a grain of salt