Boo
Hmm, well I just bought a Pixel, and am waiting for the headset to become available, so I guess I'll find out soon enough if this is a real issue. For sure, extended VR use of my Nexus 5 with Cardboard leaves it very hot and crashy, but I kind of expected that since Pixel has actually been designed for VR use, that that wouldn't be such an issue.
On the plus side, it's winter, and the advantage of phone-based VR is you aren't tethered to a PC, so I guess I can always go outside and freeze my nuts off to keep the phone cool.