Speaking for the US here:
Too much of a tendency for cities to treat an MUP as an infinitely long no-rules park instead of a thoroughfare. Around here MUPs are the gathering spot for dogs with 20' flexy leashes, off leash dogs, old people walking 5 wide, every parent's crotch fruit strung out everywhere on scooters and laying on the ground, joggers with headphones all over like a zig zap, and the wussy fake paceline cyclists treating it like the TdF.
The problem when it's "for everyone" is that nobody can seem to respect the rules that allow some semblance of order. When everyone is priority, nobody wins.
I'm more in favor of restricted use trails or trails that give priority to certain users.