Because of his public clowning persona, Bruno gets very underrated as a boxer. His only defeats came to Tyson, Lewis, Witherspoon(all HW title fights), and Smith(an early mistake). He had great power, and a wicked jab. Along with his power he had good skills; he was totally out boxing Lewis till he got caught.
Bruno hit VERY hard. He visibly stunned Tyson and was the first to do so, and like people had mentioned he hit McCall as hard as anyone ever did. His KO percentage speaks for itself, as well.
I'm not sure how much Bruno focused on flexibility but it seemed like he was stiff and robotic in the ring like you say. One of the things that Holyfield did well when he bulked up from the cruisers was to include plenty of flexibility routines in his training. I think to sum things up Bruno was never really a natural boxer - he did the fundamentals quite well but it all looked forced. But no debate about his raw power.