For email, designed like it's the 90s is a good thing, in my opinion. Thunderbird could do with a bit of polish, sure, but it has every option under the sun. I tried Geary once, it looked shiny, but the interface was far too basic.

Of all problems Thunderbird has, the look and feel of the UI is the least of my problems actually. Maybe it's just how I use mail, but probably like 90% of interaction with the UI is that white space where I read and type email text. I don't really care, or even notice, what's shown around that white space.

Why is barebone UI a problem? What feature a UI looking like it's written in the 90's is preventing?

That's a feature, not a bug.