I agree on all counts. I have machines using all three graphics drivers, and the state of play, so to speak, is abysmal. Linux can't even play full screen HD Hulu on an ATI 4850 video card without stuttering. While we're on the subject, audio is in just as sad a state. I've been trying to use Linux for prosumer level audio recording for a dozen years, about once a year, and every single time I've gone wimpering back to Windows for audio work. So, yeah, the state of multimedia on Linux, while dramatically better than its ever been, is still embarrassingly bad.
So, yeah, I probably wouldn't want to be the developer tasked with making Unity work on Linux (even a very limited subset, like "just nVidia and ATI provided drivers"), as it would probably suck. The guys at id software managed to do high end 3D gaming on Linux in the past, but they also complained loudly and frequently about the state of things.
I wasn't suggesting I blame the Unity folks for not supporting Linux. Just saying that developers like Linux. Unity likes developers. Maybe they could hookup sometime. But, it might be more trouble than it's worth for them, and I'd understand that, too (it's why we don't have an installer for Mac OS X for Virtualmin, despite the fact that more than a handful of our users like having a development environment on their Mac at home that is similar to their Linux server). If it existed for Linux, I'd almost certainly give it a try; I've considered it in the past, but the whole rebooting thing just makes it less likely that I'd ever get around to hacking on any games.