1. the designers never considered keyboard navigation, which is now even more awkward than it was already
2. the interface is now very visibly slower - startup and general usage (on two of my systems)
This ignores the very long standing issues like:
1. searching is very confusing; there are several different types of searches
2. writing an email still requires opening another window. compose emails like it's the 90s!
Inbound email is 99.9% notifications. I read and delete most of them with K9 on my phone before downloading what's left with Thunderbird. BTW my mailbox is on a POP3 server. I clear it after downloading the messages.
Personally, I find it a bit jarring, because contextually, that bar only contains stuff related to mail, but that bar and some stuff on it continues to appear even when I switch to Calendar. Thankfully, some CSS takes care of this.
Otherwise, it is a quite nice change. Dark mode now also applies to mail contents as well, which is a great change, though I would like a shortcut to swap to light mode.