" Will it sustain a hard impact or fail?"
Yes, I beat the crap out of mine, and it has never lost its zero or failed in any manner. I replace my batteries every few months whether I need to or not, and carry a supply of them with me. I real world use mine and can't afford for it to go out. I was laying in sticker bushes for almost an hour on a baricaded suspect call where he had been shooting at people (last night). I had 100% confidence in my equipment that if I needed to make the shot that I would get the hit.
A buddy of mine just came back from Iraq (SF team leader), where he was using the Aimpoint. He commented that the guys on the teams felt they were old technology and liked their EOTechs better.
To each their own, but for CQB up to 100 yard engagements, I feel the EOTech is the better tool.