Okay, here's my 2 cents.

In New York City and Westchester, the MUPs are so crowded, that anything approaching real cycling speed is as much a hazard to pedestrians, joggers, parents teaching their kids to ride bicycles, etc., as motorists are to cyclists on the roads. Cyclists are the motorists of the MUP.

This wasn't true when they were first built, and cyclists and the odd skater were the only users. Now they're so popular, as a park or open space alternative, that they are rapidly becoming untenable as bike routes.