An MUP path is barely one step above a wide open mega-store parking lot (pick a brand, Wally, Home Depot, Lowes etc) when it comes to people's behavior & predictability. Drivers who generally follow the rules of the road and don't perform risky maneuvers will suddenly drive their car like it was a jet ski on a lake as if no one else exists. Bike paths/MUPs are a lot like that. That said, I believe having a place for fledgling cyclists is a great idea but lumping in runners, walkers, pets and even horses into the same 12'-14' corridor is nothing but a a recipe for disaster. Personally, I avoid them because I don't want to slow down to a walking pace so I can tap a walker/runner on the shoulder who has earphones in or deal with 3-4 walkers walking side by side blocking the entire path talking and are completely oblivious to the fact that someone else is on the planet. As mentioned, pets on long completely extended leashes are another hazard. About the only way I'll use the MUP is when weather conditions (rain, cold etc) keep 99.9% of people off them; even then, I make sure it's a week day.
The only real knock I have with MUPs is when a path exists within a linear mile of where I'm riding on the road, car drivers feel the obligation to scream out the window that I should be on the bike path. In the end, I guess my beef is with stupid people and not MUPs.
That's all I got.