They get too hot when braking hard for too long. This can cook the grease in the bearings. A steep long hill can be a problem. You might have to repack the hubs after one long hill.
Thus the name "Repack Hill" when MTB's were just modified fat tire bikes. The ones with coaster brakes had to be repacked after one downhill ride I believe. Rim brakes are a lot better. However in the city on mostly flat roads, I don't see a problem with overheating but they don't stop fast. Compared to a bike with rim brakes front and rear you lose a lot more than half your stopping power. If you are careful you can get used to it. If you want to go very fast in traffic you would be much better off with two rim brakes. I have a few coaster brake bikes and I love them, but I keep off the long steep downhills.
Just going fast is fine, it's the hard braking that's a problem.
I built one with 700c wheels and thin tires, I love it.