Nice writeup!
Context: the primary use of personal email, for me, is conversational threads with friends and family. I don’t run business out of it and I don’t subscribe to a ton of newsletters.
Hey’s working out quite well for me. It’s just my personal email at this point, so I don’t think about it as often as I used to. I actually don’t think about email in general as much as I used to, because Hey reduced a bunch of problems for me, in a way that requires zero maintenance, and it also replaced my old solutions that had been running for ~15 years in Gmail without my having to deal with cruft anymore. More time to figure out to-do organization. 
I also really like the aesthetic though understand that’s subjective.
It seems like one difference between me and everyone else in the thread is that I actually like having the new email address, i.e. I’m not just handing out my old email address and processing in Hey.
My biggest issue was handling huge threads that go on for days or weeks, which I’ve written about elsewhere on MPU. I love these email threads because some of my best friends keep up with each other this way. It took a few months, but they finally implemented a solution that makes it easy to stay caught up with them without loading the whole thread. That eliminated my biggest concern about sticking with Hey.
My second biggest issue is search. You’re right about all its limitations although, for me, the way it surfaces files mitigates that as it’s saved me time a few times. The most annoying search term I’ve found is “D&D” - it can’t find my D&D threads! Support has acknowledged it needs improvement but not given a timeline.
The third biggest problem is the performance of the Mac app–hopefully electron support for Apple Silicon trickles down into production apps quickly. In the meantime, I just use the browser. The iOS app is much better and the bulk of my usage.
So, again, nice writeup, totally agree, and at the same time, I would hate to give up the saved time, peace of mind, and fun I’m having since the email address switch.