This is definitely something that UT needs - polish. This is awesome and hopefully this becomes a successful initiative
On top of my head, these are the things that I think matters for most people.
Display cutout and rounded corners handling. Most phones now have them and it just makes UT look unfinished and unpolished and of course annoying for the users
AGPS. Without it, navigation is really hard to rely on. It may work quick especially if you use it often but it's still highly unreliable.
Cloud syncing for contacts, notes, etc. Ideally, an easy to use backup/restore solution as well.
Video playback. I think one of the weakest part of UT since the beginning is video playback. It never worked well. It has minor issues like video flipping at the end of playback then some major issues like actually crashing the app, for example on Teleports. There are also devices where it totally stop working until you restart media hub or rebooting the device. The experience is just bad in general in my opinion
Small back button in apps and hard to reach in one hand use. Especially if you come from Android, I think you can really be annoyed by this and more especially if you are someone who frequently use your phone with one hand. This has been discussed dozens of times but it's a tricky thing to solve. I think adding a universal back action like Android is out of the question but what else can we do? I have some solutions on my apps and in Ambot Installer calles MariKit. It adds horizontal swipe at the bottom edge which can used to trigger back action or other header actions. It's very useful in my opinion but I'm still unsure if this is the best solution and approach.