The MT box is a very robust box. A large surface area anchored into the board. A large anchor on the fins too. A single large screw holding the fin in to make for a solid assembly. But the fin assortment is limited. My last set came from NSP.
I never broke a MT fin box but I have broken plenty of fins. They shear even to the bottom of the board at the fin base. I think I would have lost many more if it were not for this solid assembly. I think it allows the fin to take on some good impacts while not losing the fin or ripping the box out of the board.
That said, I surf fresh water and prefer the the ride from the cheaper, softer plastic fins. The ride was too stiff /tight with the stiffer fins that I have tried. I think the cheaper plastic fin may work a little better with this box as the flex is more in the fins design and not so much at the base of the fin. The FCS system for example have a very tiny base which allows more flex under load at the base so rides better with stiffer fins (just a theory ).