Many people are too pessimistic. Sure if this was a new company I would be a lot more worried. Sure GAW is not being transparent with zenpool but they have a great track record. A lot of the competitors are now following (look at BITMAIN) so it means this new idea is pretty good and is making profits for the hosting company. It does not mean this is a scam. ROI is a different story though
Bitmain makes their own asics and everyone knows it since they sell them. Gaw doesn't make their own asics. Kind of a big difference. But you are right, cloud mining seems to be gaining a lot of traction at the moment.
This might not be true for too much longer.