yeah that was one of my warnings when the Ube/Plum was commonly talked about; that and the nature of Wifi in how even if the devices have very little to "say", the traditional access point won't handle more than 30-50 devices connected to it MAX and it'll seriously slow down the overall wifi infrastructure because the radio still has to dedicate time to address every connected device, and the more you connect, the less time there is for all this "talking" to happen. Carving out a separate SSID on an existing AP will do nothing to help this, as even multi-SSID AP's still have to split the time with the same radio. You can add more AP's but then you get into the fun game of RF engineering to keep hotspots small and low powered and staggering them appropriately so you get adequate coverage in a small cell without creating interference between the other AP's given the very limited selection of non-overlapping channels.
If you can handle all that though, throwing together a separate subnet just for your lighting should be easy enough to accomplish. Besides, these mFi devices are really geared toward commercial spaces first; we'll see if they ever mature enough to be viable for home use. That said there are plenty of hardcore ubiquiti fans on their forum that try to shoehorn mFi devices in so many places where there are already much cheaper and more established solutions, so who knows which direction they'll push.