It's a shame to hear you're leaving Tabby behind
RoundCube isn't really stunted, it's more of a time thing. No-one gets paid for it. I do know that development is still progressing though. Matter of fact, I was just contacted earlier this week by Thomas about the project. So it's still progressing, it's just slow due to time constraints.
There are only 2 more tickets left (one of which I know is probably 75% done) and the other should be right along before 0.1 stable is released. So the project has been going, I just don't think they've been releasing often. Something that might need to be changed for future releases