I would actually like to hear what you didn't like about the UI, I personally really liked it and the only part I found hard to navigate whas the Digibank so I hope the keep it for the most part the same for the next Digimon Story game.
You can only equip items on that specific screen, which is dumb because why not let us equip from the Items screen? You have to go to two screens instead of one.
You can't transfer items from one Digimon to another, this is a pain in the ass because you have to go in and out a lot of screens to prepare your team.
If you want to reasign your Digimon moves, again you have to go through more screens than you should and that's a pain in the ass. Why no do it directly on the screen you can check on them by pressing triangle and right directional?
If you DNA divivolve you lose one perfectly fine and trained Digimon forever if you want to de-digivolve. That is one asnine decision.
Some of the quests barely give you an indication about what you should do and when you talk to Mirei, she doesn't help at all.
Also in regards to the quests, sometimes you are teleported directly to who contracted you, sometimes not. This makes no sense at all, why not do it always? Why do I have to waste time walking back and forth like an idiot? If
Most of the Megas are utterly useless, since most don't have a piercing attack. At the end of the game, together with a healing/buffer Digimon, they are a must have. You won't beat eaters without them, you won't beat the colosseum without them.
Also, why most of them are virus? This was really badly distributed, also we have one data Digimon with a piercing attack which is ridiculous.
OH, and non-skippable cutscenes is an unforgivable sin in 2016.
These are thing that bothered that I can remember right now.