Roundcube indeed does not have all OWM features built-in (like Calendar or webdisk), but plugins exist. To have them installed and properly integrated you'd need to install a groupware system, e.g. Kolab. I'm sure there are features in Roundcube that OWM doesn't have, but I never did a comparison. Please also consider the fact that OWM is dying, but Roundcube is doing well, has a commercial support and some developers are paid for working on it.
