The menus are definitely busy and confusing (there are many options), but once it's set up I've never been bothered by the UI. I quite like how emails are shown OOTB in fact, with the right padding and day separation; I also use most buttons that are offered by default. So yeah, sane defaults.

Off the top of my head what I like:

• you can search emails on the server! That's the one feature I couldn't forgo
• it detects unsubscribe links
• I remember having delays when using K9, with FairEmail not once. This could be fixed now
• it cleverly shows you're connected to a VPN (which can be a problem with emails)
• maybe too many options, but a lot are actually useful

Not trying to say it's better, to each their own. But it's great.

edit: I received an email at 06:19 in fairemail. it's now 06:56 and I just received it in thunderbird