I used Google Fi last month in China and it worked great. I didn't have any VPN installed and I was using google's VPN that comes with Google Fi and had no issue with any US sites. The only major issue I had was uploading pictures or video on google hangout. Uploading to instagram had no issues, so I wonder why hangout gives me trouble.

I had no issue at all in Shanghai.
Xi'an I had no issue but the google map locations for stuff is way off so don't rely on it.
Beijing I occasionally get dropped and I try to manually connect to another carrier and it works again. I had issues with google map labeling my hotel about a mile away from where it is actually so I would avoid relying on the map locations too much.