Like you, I was also very worried about using Canon lenses on a Sony. I read a lot of reviews, and was actually very skeptical. The issues that you mention make sense. Using an adapter is probably a "risky business", with mechanical problems (non planar adapter, tightness, etc).
Probably I was lucky. My adapter is nice and tight (and the beginning, very tight), and I can't see any asymmetry in the images. Taking the lens off definitely requires more force than on the native Canon. My 24-70 f/2.8II looks sharp side to side on the Sony.
As to the shutter shock on the A7R. It's likely there, but I haven't seen it. I am sure one can find a shutter speed / lens combo, where some sharpness is lost due to this effect. Perhaps even to the level of going from 36Mpix resolution to 20Mpix ? But you still have that Sony Dynamic Range. Again, perhaps I have been lucky.
So just make sure you can return the Metabones if it's out of whack, and try a few from different batches. Last resort, you can get the Zeiss 24-70mm f/4 for the Sony, which isn't bad at all (I hear, I'm happy with the my "CanSony" combo - it was my backup plan, if the Metabones turned out to be a dud). All of this is still cheaper than the Nikon+24-70 combo, which I considered also before going the A7R route.
Good luck :-)