Earlier quoted context omitted.
I'm with you on this. I tried Thunderbird again just a few weeks back, to see if it could replace Outlook for me. Nope, but it's nothing to do with the UI, which seems... fine? My issues were: - no support for O365, unless you pay for a 3rd party plugin - bizarrely high CPU usage, even when seemingly not doing anything - *sometimes* memory usage grows really high - a bunch of small niggling issues over missing featur…
I would add to that GPG just doesn’t work. They have support for it but if you try to use it fails to store any keys.
Since its deprecation and Thunderbird's integration of GPG, there were a few hiccups (mostly UI differences between Enigmail and native TB support), but none with key management.
I can generate, store and use keys without issue.