As far as I understood Mailpile is a mail user agent, so not comparable. Roundcube runs on a server, Mailpile runs on your own computer.

"You can [then] access it over the network using your web browser" https://www.mailpile.is/faq/#wha-2 seems to challenge your knowledge. I don't know if using it so compromises privacy in unexpected ways, or indeed even works satisfactorily; I am not a user.

It now indeed seems to be the planned path to create a full webmail service out of Mailpile. Though right now the developers still say its "written as a personal application": https://www.mailpile.is/faq/#tec-8

It's designed to run on your local machine and be accessed by your browser. You can run it on a server if you like, but the give-away is the fact that it only supports single users — the original conception was for personal use.

I heard they're moving in the direction of supporting multiple users, which will be fantastic. Then you really can use it as a proper webmail client for a small email server.

I will be doing that as soon the 1.0 release is out and available as a debian package. Any idea how stable the current beta version is?