FWIW, I successfully used my unofficially supported Galaxy S9 in India. I did hit one snag, which is that I had bought the phone prior to going on the trip and hadn't installed the Project Fi app yet which meant I couldn't connect to any network after landing. Once I got on wifi and installed the app, I had 4G during my entire stay even in the most rural areas on overnight trains and no issues.
Though had I bothered to go through the whole passport photo/scan 24hr submission process for Airtel it would have been way cheaper.
When I brought my Samsung S7 with a Fi data sim to Korea, it worked fine. When I brought it to Japan, nothing. My Nexus 5X worked great in both places.
I believe that's because there US S7 doesn't have the LTE receiver for the data band used in Japan, whereas the 5X does and not the issue of the Fi SIM. I bet if you got an S7 from Japan it would have worked fine
I'm not sure if it would have been better on a supported phone, but in the past year I've had good coverage with an iPhone SE on Fi in New Zealand, Switzerland, Germany, and Austria.
No, it shouldn't, and I hope Google isn't lazy enough to add an unnecessary limitation. T-Mobile's very similar roaming (which Google's seems to be based off of) works fine with unlocked non-T-Mobile branded phones.
Though had I bothered to go through the whole passport photo/scan 24hr submission process for Airtel it would have been way cheaper.