Hello Rob,
If I am well informed, nginx is only used for the Admin Webserver. The Webserver, Roundcube Ubuntu packages are depending on, is Apache. So that isn't a real way to go. There is nothing wrong with SOGo / OpenCHange webmail. But more users will find out, It's only supporting one sending domain. And users are more familiar with Roundcube, true.
Thanks,
Frans van Berckel