I've never had any problems with that at all, it's not always you get stunned if someone blocks - I think it's several factors that lead to that you're getting stunned, timing of when you/they bring their shield or weapon up for a block and what attack you're currently using...
But I don't think they're too good to block, a power attack usually pwns the weaker creatures like goblins and such, just overwhelm them with power and send the straight to the ground - Usually that's what happens with when I play...
If you think they are too good, just use the difficulty slider...
