I have heard complaints where I work that less-technical foks find the UI unpleasant/confusing. I think it is due to a combination of the keyboard shortcuts being front-and-center and the threading model seeming "complex" (really, just logically structured)

I'm not sure there is a strong/good argument against it, though. e.g. word processors are "more complex" than text files for sure, but people have learned the benefits of those.

I think the education / user-onboarding flow could be improved perhaps.