I think maybe the original killer app for microwaves was baked potatoes? An hour to cook in a conventional oven. 5 minutes in a microwave. But maybe no one eats those anymore?
I haven't tried that, but my guess would be the same problem as most solids in a microwave - uneven heating / cold spots. That's why liquids and popcorn work so well, liquids mix themselves up and the unpopped kernels fall to the bottom of the bag.
I have one (800w) that takes about 5min to cook a potato (200gr), the manual suggests "once the potatoes are cooked, wrap them in tin foil for at least 5 minutes to cook through" but I just cook one wrapped with baking paper.
