It is bug ridden, with a long standing crufty code base nobody really knows how to interact with. It is getting some love nowadays, yes, but I think it will take some ages for it to be modern enough to have more devs willing to work on it.

Also TB does not support conversations (think of GMail), so you don't a full thread view. Pointless for operation.

I still use it, but just because Outlook is far worse.