Can’t speak for others but for me Thunderbird’s UI feels distracting and almost grating in some ways. It’s technically functional but doesn’t feel right.

I know things like that don’t bother some folks and that’s fine, but others like myself can’t turn the visual parts of their brains off and so some software feels like nails on a chalkboard to use.

The search is also incredibly slow. I didn't notice just how slow and inaccurate it is until I built my mail client on notmuch.