On MacRumors [1], the developer responded in a thread and mentioned that its just him, no big team. So, perhaps he's just overwhelmed with customer queries?
That said, Duet works well on my 13" Early 2011 MBP and a iPad Mini Retina, normal resolution. Retina resolution bumps up the cpu utilization. Duet does not work so well with the same machine connected to an iPad 3, but still usable. Duet does NOT work with 10.10.2 BETA according to the MacRumors thread. Hoping for updates.
[1] http://www.macrumors.com/2014/12/18/duet-display-ipad-tether...