Two main reasons come to my mind as to why a roadway or path doesn't work well for travelers: poor design, and/or user error.
On MUPs, I normally see a variety of people who are utterly, blissfully, even recklessly, unaware that others are attempting legitimate use of that same stretch of path. A good number of pedestrians act as though nobody else is going to come along at greater than walking speed, or they'll take up the full width of the path and effectively block use by anyone else. A good number of riders of other modes of transportation (bikes, boards) will also meander around on the path in a manner oblivious to others coming along at different speeds.
Above and beyond whatever design characteristics contribute, such as when they cross or come near roadways with motorized vehicles, or cross zones with large numbers of pedestrians.
The "user error" issue can be easily solved, one participant at a time. Of course, that'd take a bit of thinking, a bit of inconvenience, and a bit of sharing on the part of all who use such paths. In my experience, while the great percentage of all who use such paths are pretty good about it, there's a percentage who likely never will be.
And so, for me, when I use the local MUPs I tend to go along at a fairly safe speed differential, tend to stay more-or-less glued to the RH side of the path (until it's time to pass someone), and tend to telegraph my intended moves far in advance. It's been decades since I've had a mishap on a MUP ... that being an incident in which a pedestrian was oblivious to the fact of crossing a MUP with oncoming traffic flow. (I was the nearest "traffic flow" ... and went down because of it.)
All things considered, I prefer MUPs to motor vehicle roadways. I'd rather tangle with a hesitant pedestrian than a car with blind spots that's ~4K pounds heavier than I.