My main gripe with the Fastmail client is that it doesn't work offline. This is of course absolutely possible to do with a webapp, and IMO ought to be a priority for an email client.
I literally switched on "Enable offline support", caching "All mail" offline on my iPhone a few months ago. Tons of free space, only using 4GB for offline.
But when my phone is actually offline (on a plane or elevator) it beachballs when trying to find something.
I didn't, but I'll do so now. I usually don't bother with support, because I forget that good companies like Fastmail actually have a competent support team.
Not OP, but I had to change the mail cache from all mail to recent, I think because it couldn't handle how much mail I had? It would often get stuck not syncing, and offline would be unreliable and showing a blank screen. Recent works fine, but it would be worthwhile folks testing more with all mail enabled, especially if they have larger inboxes.