I've had 74km/h on my Mini, and 82 on the Swift, and routinely got over 70 on the R20 (until I changed my commute route).

There is a very serious problem with speed on a bicycle; a sudden inadvertent steering movement may initiate speed wobbles, also called the death wobble. It is very difficult to recover from this and easy to crash. But as far as I know, this is the only danger with going fast (other than misjudging a corner or brakes fading or something obvious like that). Of course, there is also the danger of the tyre failing if it is in poor condition or a crappy quality.