That's just a matter of personal judgment. It's like driving your car 90 mph, is it safe or not? Personally, I'm a chicken and if I wanted to ride motorcycle speeds, I'd want motorcycle wheels under me and a motorcycle helmet and leathers. So I try to hold it down on the descents. But lots of people enjoy going down them fast.

I don't know that the style of bike makes that much difference. A bike intended for speed is going to be lighter than others, not necessarily stronger. On the Worksman site, they show industrial cruiser bikes being used to descend the volcano in Maui (bus takes you up, you coast down on a bike). And those bikes are about as unroadbikish as you can get.