I switched back a few times but it keeps doing it automatically more and more frequently, and the new app is absolutely crippled and hideously slow (all operations are over the network and the latency is insane).
I just switched back to Thunderbird (which I haven't used since the 00s) instead. It's still ugly, but it isn't getting in my way.
Default choice of fonts, mishmash of visual styles from across decades of development in the various widgets, non-native/non-pixel-aligned font rendering, context menus that relegate the most common actions to submenus while elevating actions that you're not going to use as much, alignment issues abound, use of modal dialogs for things that are taking place in the background and should be progress bar widgets instead, dialogs that need focus being placed under dialogs that can't be used until the backgrounded dialog is interacted with, etc.
It's dense, and requires a pixel perfect pointing device to use.
In other words, it's a desktop app, through and through.
If you're used to stuff that's mobile-first, it's a rather jarring change - I can see how one would consider it ugly. I happen to find it "dense, usable, and does the job fine." I wouldn't call it pretty, but neither do I mind a utilitarian interface to do things like "read my mail."
Those weren't my reasons, actually — I am old-school and prefer information-dense layouts designed for use with a mouse (though Thunderbird needs a lot of work to be usable via keyboard). I posted another comment with my actual reasons.
I just switched back to Thunderbird (which I haven't used since the 00s) instead. It's still ugly, but it isn't getting in my way.