Usability of Kicad GUI is very questionable.
Except a lot of engineers are able to, you know, design complex boards in Kicad. The GUI could be better, it has been a lot worse, but to call it "questionable" means you haven't bothered to spend any time learning how to actually use the software.
”Usability” means “ease of use”, no less, no more. A tool with low usability can still produce top-notch results, because
effectiveness is a separate thing from usability.
And yes, like basically every major open-source desktop app, KiCad’s usability is not great. The FOSS model plain and simply does not lend itself towards high usability (because that requires consistency and adherence to rules, which really is antithetical to the FOSS model).
Source: I used to be a usability designer for software and web stuff.