There is very little wrong with usability in the sense of the UI. However, in LFS you can change graphical options, and see the results immediately. I understand this is no easy task from the coding side, I've had issues myself here. However it is possible, and more user friendly when the user can change any option, and immediately see the results without needing to restart, load a track/level and get to a point where they can compare the results... Likely by that time they forgot the previous visuals.
iRacing was a pain, I never got it setup in 3 monitors because of this style of UI. It is too much of a hassle to reload the entire game and track just to see if it is working yet. Only to find out you need to restart again. Granted, the loading process for NetKar has been a little better, though it is still the annoying process, and I have yet to try setting up my three monitors there - just since I haven't really played it for more than two hours.
Is there anything directly wrong with the UI? No, you can setup the audio, visual and control options quite well - although some visual options (my native screen resolution) does not exist and is a flaw with that part of the UI. Though it is usable, easy to understand. It is just a pain in the butt.
So I agree, there is very little wrong with the actual interface, besides the fact that it is outside the game and it requires several steps to get launched. This comes with one positive, it makes you feel more like you are about to go to the track for a weekend / test day, but it comes at an inconvenience of quickly hopping in a car, or setting things up to perform exactly how you want. I mean, think of it this way: On track going around turn-1 and you realize the engine noise is too loud. You need to leave just to change the sound settings. Come back and now its too quiet. Immediate feedback while changing options is good, at the cost of development time and preparation.