To me riding on a sidewalk is a big no no. Other people may think it's okay. What I don't like about bicycles being ridden on sidewalks is that 1, the bicycle can be far less visible to motorists than if the bicycle is on the road. 1b, a bicycle on t he sidewalk is often totally out of a motorist's field of vision. 2. Bicycles being ridden on a sidewalk are often (too often in fact) being ridden at a speed far more suited to be on the road and thus are a hazard to any other sidewalk users. 3. many sidewalks have very poor sight-lines for a bicyclist approaching an intersections and driveways should be treated as intersections. 4. Riding into a major intersection from a sidewalk is very much like playing Russian Roulette but using a bicycle instead of a gun = your chances of being seriously injured are quite high. That's because you're often blocked for motorists' view by parked vehicles, motorists don't allow for the speed of a bicycle and motorists simply aren't expecting a bicycle to enter an intersection from the sidewalk. 5. Ride counter-flow to traffic whilst riding on a sidewalk great increases the danger level. 6. Then there's the possibility of hitting a pedestrian who steps out of a doorway onto the sidewalk. Plus, pedestrians are surprisingly unpredictable.

Those are some of the reasons why I avoid sidewalks when riding my bicycle.
7. Around here, in winter sidewalks are a LOT more treacherous due to snow and ice than the roads are. Snow melts in the daytime and the liquid freezes at night as the liquid has been trapped by a snowbank on the street side of the sidewalk and lawns on the building side. The sidewalk acts as a nice little canal. I find it far easier to ride on the roads than to ride on a sidewalk then. YMMV

Cheers