I have two. One is extremely petty (and a little cringe), the other is something a bit more reasonable.

The first one is the lack of international and obscure IFE. I'd kill for a good anime series or obscurely directed film (like Fallen Angels). If UA had San Gatsu no Lion in their system (or an equivalent series), I would fly them every time. I was so happy when Kimisomu was in the system earlier this year (even if it was the live action).

The second one is the fact that as a 50K Aeroplan member, checking in via the United App or online is an absolute pain, so when I have partner travel on UA, I can't even put in contact information for whatever reason without it giving an error because I'm using an Aeroplan number.