Streets sometimes have maintenance problems, but people complain if it gets too bad. But they don't always worry about the sidewalk condition. It can get really really bumpy and no one cares. Especially if you have clay soils, part of the sidewalk can go up and part goes down and you get a nice ledge effect. These sidewalks were intended to have smooth transitions from sidewalk to curb, but it doesn't always work out. Not good for bicycles, unicycles, or wheelchairs.
Parts that ought to be smooth aren't anymore.
In addition to the little ledges, you can also get some slots, but they are at least close to the curb.
Sometimes they don't even try to make the smooth transitions at sidewalks. You can cross 30 driveways and side streets and everyone will have a roll-over transition, and then you come to one that doesn't.
Sometimes, the sidewalk just gets blocked off by different stuff. Here's some bushes growing out over a sidewalk. And a nice bump in the way, and the whole sidewalk is narrow. The problem is, this road is also a terrible place to ride with heavy fast traffic. Sometimes, it's just better to find a different route.
There are a very few places where maybe a regular sidewalk is a better place to ride. This might be one of them. There aren't any driveways or cross-streets for a while, so you can't get right-hooked. And traffic on the road is fast and fairly heavy. But even on this sidewalk, you'd have to take it slow so you didn't run into one of the problems above.
(The End)