Blocked shots do not equate to good defense. In Stevens' system a big has to do more than hang around the paint looking for blocks. You have to be able to switch, even off ball, and do it right. You have to channel drivers to the basket in certain directions. You have to be willing to close out hard on three point shooters to affect them. You have to move low post players physically out of their comfort zones. And there is more.
Williams showed he can block shots and play good one on one low post defense. But he was often lost when it came to most other things. Couple that with his only showing he could dunk on offense and looking even more lost on that end. Williams needs to develop a bunch before getting any serious run.
Is Stevens' defensive system any good though? That's the real question. I personally don't think it is, which is why any team that runs major pick and roll sets will shred this defense to pieces.
That seems like a strange question, but all our big guys get exploited (even Horford, when he plays center position) in one way or another. Stevens wants all his players to be able to switch on the perimeter (and in the post), that's something you can't ask from a true center of point guard. You have to make adjustments to cover each other's weaknesses and let them play to their strength. I want my center around the basket and have drivers to think twice about trying to finish at the rim.