I switched to from Thunderbird to Betterbird several months ago when Thunderbird screwed up their unified folder view. Since it's a fork, I had no problems with compatibility. It just worked, I was happy.
A few weeks ago, I switched back to Thunderbird. Why? Betterbird started down the path of fixing problems due to Betterbird - and not Thunderbird. So, things came full circle and they now have to maintain their own stuff on top of Thunderbird.
Thunderbird has since fixed their unified folder view, so I switched back.
Read their "Software Quality" explanation at the bottom of the Betterbird home page. Product bashing is not a good long-term marking approach and does not give one confidence about this software.
I suspect that the employees hired by Thunderbird have now gotten up to speed and the issues that the Betterbird guys are complaining about are no longer a concern. That's my guess.