I really love Thunderbird; I've been using it for at least a decade now. There are definitely places where it still needs improvement, after years and years. While Betterbird seems exciting in theory, the attitude of the project leader (who it seems is a banned former contributor to Mozilla Thunderbird), and honestly even the arrogance in the name itself gives me pause. The issues laid out in this[0] email thread are…
As for Thunderbird: I use it, but it seems like its dev decisions are often wedged in rather basic ways. I have no idea how the decisions were made, but if you look at the support forum, there is a big disconnect between what the devs keep adding, and the existing pain points that users keep running into. I'd rather have a less featureful program where the dev effort went into eliminating user pain instead of adding more.