I used thunderbird for years, gave up, and then tried it again a couple years ago. I think the UI/UX is really bad and was the reason I had forgotten why I left, and it was the reason I gave up the second time.

I unfortunately can't remember my complaints to make them more specific for a lively discussion, but I guess I am an anecdotal counter-point for thinking the UX was actually not a success.

I'd say a bad UI that is stable and familiar to the majority of users is actually a better UX than some new thing. See the MS Office "ribbon" interface for a good example.

Car manufacturers don't try to reinvent the steering wheel & pedals UI every couple years.

That is basically the motto of the Bloomberg Terminal, so I would agree with that to some extent.

Search is really bad and every time I use it I feel something is now shown (and what IS shown is totally irrelevant and old).

Also, I see that a dark mode would be a requirement today as everything else either complies with OS's dark mode setting (Windows 10, OS X) or has it's own dark theme.

People say here that the UI is bad but nobody says what exactly is supposed to be bad about it. Or is it just enough that it doesn't look like the blown up UIs that are in fashion today?

The icons are ugly, there are too many boxy sections on any given view, the top left pane in the calendar provides no useful information and is ugly. There is too much noise and clutter, it could be simpler and cleaner. I use it every day, but I wish it was less ugly.

I gave up because the search was inexplicable Reddit level bad.