People typically use Mu because Notmuch was not very well documented. So the typical newbie came to Notmuch, found no way to delete a message and got frustrated

People typically use Mu because Notmuch was not very well documented. So the typical newbie came to Notmuch, found no way to delete a message and got frustrated. I think there is more or less feature parity. However, out of the Emacs world, Notmuch is very very popular as a Mutt search backend.