I had used /e/ as a daily driver for about a year, on one of the officially supported phones (FP3). Installation was easy, and coming from other custom ROMs I remember thinking "wow, this just works!” ;) Didn't use any of the /e/ services, things worked fine connected to a self-hosted nextcloud.
For a usability I think this is how painless an alternative ROM should be if it wants to reach a wider audience.
Shout-out to mailcow-dockerized, a GPL-3 licensed setup of postfix/dovecot etc with sogo as webmail. Managed by a German IT company, I've been running it in production for more than a year, serving a handful of domains. Very happy with it.
https://mailcow.email/
https://github.com/mailcow/mailcow-dockerized
Oh, and they're on mastodon as well: https://mailcow.social/@doncow