OP, I've found just the opposite. It's much easier for me to do my workouts on the hills. I can just go faster or slower, depending on what power level I'm targeting.

On the flats, there are two problems. One, I hit a lot of stop signs and stop lights. Two, for some reason, it's hard for me to maintain a power level. I get up to some target level, but after a few minutes, I've fallen off. This doesn't happen on the hills.