The only reasons I sometime back looked into betterbird was thunderbird breaking TbSync and its companion "Provider for Exchange ActiveSync", which I really need for work, and because of their tray support (I don't like the modern way which rejects the benefits of the tray functionality, or notification area which is how it's also called now a days).

For the first thing, I was able to live with thunderbird by reverting the upgrade and keep its package from upgrading at all, until the two extensions I required eventually supported the new thunderbird version which broke them. I looked into betterbird as an alternative since someone suggested it given betterbird wasn't moving as fast at that time as thunderbird was, and at that moment they were not breaking the extensions I'm force to use if wanting to use thunderbird as email client at work.

For the tray, ohh well, it doesn't work on wayland if you don't use gnome or kde (I use wayfire), so it couldn't help me at all. I found a bug reported on mozilla (not sure why not also on betterbird) which matches my case, so no luck with their tray support, :(

Other than that I really didn't find a compelling reason to use betterbird instead of thunderbird. But if I were a gnome or kde user, perhaps its tray support might be compelling enough.