Haven't had any issues with overheating thus far; I'm sort of inclined in believing that Google has done extensive testing on the CC hardware. I played D2 for an hour or so without a single hiccup--something that actually surprised me pleasantly I might add.
However, I suppose if your television is getting hot, and the CCU is attached directly to one of the ports, that this could become problematic in some cases. Mine is actually not attached directly to my TV but in my enclosure that contains other equipment. This enclosure is ventilated, but does have a notable amount of other things contained within.
Man, I do really like no install times, loading screens (or very brief ones), long updates and not worrying about local performance metrics. That alone is a pretty darn good incentive for trying Stadia imo.