[nextos]

That's not true. You can still have Notmuch to play well with other mail clients. You simply need to work out a good mapping between tags and folders, which is trivial.

IMAP only supports a few tags: read, unread... But not custom ones.

The only possible scenario where Mu is better than Notmuch is where you have many many custom tags and you want them to be perfectly synced across Emacs instances in different computers. Since Mu stores them in messages, this is very easy. With Notmuch, you'll need to separately sync the database. E.g. by rsyncing it.

But note that other clients won't understand these tags.