What about difficulty - i think that now it's better to rework AI first. This will change difficulty entirely. And only after that we will see, how difficult it is, and what is possible to do to balance the game.
Anyway, as it already was told, now on UV on level 8-10 you face the most powerfull enemies you can face, and after that you can see nothing new, more difficult. It you add that the further you go, the more traits, good armor, powerups, large med-packs, and powerfull weapons and mods you have, you will see something like this:
Power of monsters
| _ __ _____
| _ _ _ _
| _ _
| _
|_
|______________________ Level
1 2 3 4 5 6 7 8 9 10 11-25
Amount of good items and powerups
| ____
| __ __ __ __
| _ _ __ __
| _ _ _
| _ _ _
|___________________________________ Level
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16-25
Power of monsters increasing very fast on first levels, but after level 8-10 it's constant. Power of items spawned increases more slightly, so when you face quite powerfull monsters, you still don't have good items and powerups, or at least traits(it's first levels, so maximum it's clvl 5-6).
It's UV, so we can't reduce power of monsters. Then we need to raise power of items and powerups for first levels. It's all MHO.
What about the rest of the game. It was the game what i always wanted. It always holds you in tense, not allowing you to relax, you still can die if you make stupid move, but it's not "oh, i don't have ammo, i don't have med-packs, what RNG will create on next level, this will determine my fate" game.
I like modability of every weapon. I already said that it's not overpowering, and not useless(damage mod for PR rules)). Rarity... I cannot say that i had a lot of mods that i couldn't use, and i cannot say that i searched for every mod for centuries.