I can't really rate Roundcube, as in the three days since my ISP made the switch, I've stopped receiving email!
The one feature that I was excited about, the ability to TURN OFF filtering, turns out to be a chimera. It just lets me avoid adding my own filters on top of those of the provider. Since the provider seems to be blocking 100% of my email, that doesn't help me any.
Email is failing as a communications system, which makes evaluations of Roundcube kind of irrelevent.