Any known issues with installing/running roundcube and dovecot on the same server?
There is a generic issue with doing this. That is if you have roundcube (or any other web mail interface) on the same server as dovecot, a breach of the web interface could be quite serious and allow access to the complete mail store.
A better configuration is to run the web mail interface on an isolated server and get it to communicate using TLS imap with a remote dovecot service.
For economy, you could do this on the same machine using a small virtual server to run roundcube
I disagree with this, and that is what user/group/permissions are for.
Roundcube does not have direct file access to the emails even on the same server. Roundcube opens a connection to dovecot, supplies the user/pass/login credentials to dovecot, and dovecot fetches the email stores and serves it to roundcube. There is nothing a hacker can gain access to by exploiting roundcube that they also couldn't get in the same scenario if roundcube and dovecot were on two different machines.