I didn't notice any difference when compared to regular small ovens. Well, I guess there is some accelerated crust build up if you let your microwaved food splatter around and then bake it in. If you repeat that cycle a few times, it will look really filthy.

Fortunately, that problem is fairly easy to solve. Use a cover if you microwave stuff which might splatter around.

Well, to be honest, you don't actually have to clear an oven regularly. That stuff has been burnt to a crisp ages ago. Heating it up won't produce any smoke or smell anymore. All of that was burnt away.