Busy MUPs can be frustratingly slow, but I certainly don't feel nervous. Maybe you're trying to bike too fast for the conditions?

The MUP I use most is a very busy bridge crossing. Lots of cyclists and pedestrians. The heights make me nervous (this bridge runs accross the top of our deep and wide river valley, which is why it's so popular), but the MUPness of it just means I have to be prepared to slow to walking speed until a passing opportunity presents itself. Biking on a MUP with pedestrians is the same situation thing as driving on a road with bikes.