I'm guessing you haven't read the book, so I don't want to spoil. But I can think of a maybe 4 or 5 big surprises, and all but one felt "earned" (in the sense of, it totally made sense and the author didn't pull it out of his ass, but at the same time was surprising).
One of the themes of Mistborn, according to Sanderson, is "there's always another secret." This, combined with his obsessive level of personal worldbuilding, means that he's especially good at this kind of thing.
Interestingly enough, it's a strength he definitely was NOT able to leverage well in WoT, since the worldbuilding and twists had been worked out ahead of time by Jordan.