I had the same reaction to the Mistborn books, and my interpretation is that Sanderson really struggles when the plot and characters get "wide". The first book was tight and focused; when the characters graduated to a larger stage everything - from the characters to the world to the plot - ended up "flat" and unrealistic. It's like he has a set budget for any given work, and the wider the focus the less of that budget any individual piece gets.
Which is a shame, because some of his work really is excellent.
I decided to try the Mistborn trilogy. The first book was great - a nice tight and satisfying story. But the second and third books got progressively messier and I found myself totally disconnected and eventually just slogging through to the end. I wish I would have stopped after the first book.
Sanderson admits loving to write write write and hating to rewrite and edit, which I think really showed in the latter books of the trilogy.