The rear fcs type boxes have worked well for me. I did have a problem with teh fins working themselves loose, but this was only when the board/fin was completely new, the first one or two outings. After that they "set" and works fine and yet I have to remove my fins between every session because of transportation issues. On one box, the screw was a bit to tight, meaning that it was difficult to feel how much pressure you put on it. But a small drip of oil solved that (olive oil actually - all I had in my camper at the time). If there is resistance int he screws even when no fin is present, I can recommend that solution since it really makes it a lot easier to get the fins tight.
There are already masses of Quad boards out on the beaches. Usually, we hear about problems quickly (on this forum fx) but there has been only one issue I can recall and that was about a lost screw, that probably went in wrong in the threads when it was replaced. Otherwise nothing. So while I also think MT in the rear would feel even sturdier (but would also be a slight weight penalty) I'm confident in my rear fins on all my quads.