Thanks, keeping this as a reference — I'm trying to find some time to try mu4e. I used Gnus for many years, then switched to Apple Mail.app, but with the gradual decline of MacOS (and Mail.app) I'm looking to switch back.
I remember the two main reasons I switched from Gnus: 1) there was no good reliable search, 2) I couldn't drag&drop attachments into E-mails and back so I was spending a lot of time pointing to files. I hope both things have improved since then.
concerning (1): I have no offline sync in place, all my emails stay on the server. The IMAP protocol has a decent server-side search included[0], combined with Gnus unified search syntax[1], I enjoy a hassle-free search experience.
gnus had some massive IMAP performance improvements a few years (probably close to a decade now) ago. Before that it was quite painful to use it on large mailboxes without a local imap - I used to sync that with offlineimap. When they had a massive issue moving from python2 to python3, and keeping that running on a modern distro started getting painful I tried it without local imap - and realised those improvements made things fast enough that you can run it on remote mailboxes, and even do so in your main emacs instance.
I remember the two main reasons I switched from Gnus: 1) there was no good reliable search, 2) I couldn't drag&drop attachments into E-mails and back so I was spending a lot of time pointing to files. I hope both things have improved since then.