I keep swinging between dislike and acceptance with Thunderbird.
It feels old, clunky and cludgy. And not in a good way (the way Vim is old, but still marvellous, or mutt is cludgy but still Just Works Everywhere).
The UI feels horribly out of place (on my vanilla Ubuntu), the UX is terribly inconsistent and after -I think- 10+ years of using it, I still have to search where to find stuff. Daily.
I've been comparing it with each and every option for the Ubuntu desktop and even webmail. Clients like Geary[1] or Roundcube [2].
Those are often unfinished; I guess mail clients are like many other software-challenges in that regard: it's easy to make the basics, the 90% that 90% of the people need. It's doable to make those basics really good (see all the webmail, electron mail and other mail clients). But it is really hard to do the last 10%, because each of the individual users has a different 10%.
And so, I find I love the UX of Geary and use Roundcube often for its simplicity and "Getting Stuff Done". But I also find that I keep going back to Thunderbird, because is only there. But, boy, it annoys me how cludgy and convoluted thunderbird then feels.
Edit: forgot to mention that a lot of the 10% things in Thunderbird are severely in my way, daily. I have an RSS reader - why is there one in Thunderbird taking up mental (and memory) space, I don't ever use the built-in IM (is it jabber?), the calendar and todo-tasks feature I don't use, but they attempt to manage my agenda/invites anyway, making me miss an occasional meeting even. I'm sure other people find the GPG being severely in the way (part of the 10% that I use often) or that archiving and offline features are clumsy, confusing and making people miss emails (another feature I often use).
--
[1] https://wiki.gnome.org/Apps/Geary
[2] https://roundcube.net/