Well it's apparently not just UI, but options in general.
Seems fov cannot be changed. Stealth is no longer a toggle but you need to keep the button pressed, normally this would be an option but alledgedly this is not the case in FO4.
You can hit hotkeys to enter menu's but not exit them with the same hotkeys. Sensitivity is different between the X and the Y axis. Mouse acceleration that alledgedly can only be turned off in the .ini's. It seems the weapon wheel is navigated by the WASD keys, so you stand still as the movement keys are now used to navigate while the game continue's. Power attacking is not on "hold fire button" anymore, but on the bash key.
The mouse scroll wheel apparently also does nothing in the pipboy navigation.
Based on criticism i've read elsewhere, these are some pretty jarring issues. UI is completely unoptimized for mouse and keyboard, and there is a lack of what can be customized in the options menu.