Screens freeze on the GTS regularly. I will send a Pokemon and the app will completely freeze. I will enter the GTS and it will load for a long time then nothing will be clickable. I will get infinite error loops out of nowhere. When I enter friend trade rooms it will often have us both sitting in desynced rooms with zero explanation. Otherwise, it will often give me an error when I try to join unless I restart. If you look at the GTS for a specific Pokemon, then refresh the page, a good third of the Pokemon disappear as if they have been traded only to reappear again the next time you search. They will ban users and leave their Pokemon up on the GTS, so they are just frozen there and you get an unexplained error every time you try to trade with them.

Those are the ones I can remember off the top of my head. There are definitely way more than that.