I heard cleaning the hood is easier if you boil some water and let the steam condense on the hood/micro.
I've noticed OTR microwaves often are dirtier than hoods, and I think it could be that often the bottom of the OTR micro is closer to the range top than a hood would be by design. Of course it could be due to people not cleaning it often enough or not realizing it's dirty.
But any surface whether it's a hood or OTR micro can have an increased fire hazard if the surfaces, filters or ducts aren't kept clean. Enough heat or a fire starts on the cooktop, and you have an ignition source for it to spread.
Some houses, that obviously had a vent hood originally, have an OTR micro installed and it's way too close to the cook top. I mean to the point where you'd have to bend down to see the controls on the range or check a pot on the back burner.
Our house, the cabinet people suggested a shorter cabinet above the range in case we ever installed an OTR micro, which will never happen as there's plenty of counter space to not need one. Due to that shorter cabinet the vent hood is high enough that I could almost stand under it if the range wasn't sitting below. In that case it probably won't be as effective as if it's closer to the cooktop.
