I can't speak on the Steam version, but the UI was perfectly fine in the ASCII version. The worst thing that I remember was how things were categorized. IE it was something like (b)uildings, (q)ueries, (d)esignations and whatever (i) was, got weirdly split apart. I forget the exact nature of it because it's been so long since I last played. But that seemed more a quirk of the game's spaghetti code ontological structure, because these things did have genuinely different behaviors despite being conceptually similar. That's a problem at a deeper layer than the UI/UX.
The only other thing I can think of is the labor menu was kind of annoying for large fortresses.
They fixed all that. Go give the free version another try; it has the updated UI but not the new sprites so it feels a lot like playing the older versions of the game but with more polish.
The only other thing I can think of is the labor menu was kind of annoying for large fortresses.