I was having issues with T Mobile, where my phone (5X) would always default to T Mobile at my house, yet the signal was not strong enough to place or receive a call. It would show about half strength LTE right up until I tried to place a call then it would drop to nothing, and the call would fail. Most incoming calls never arrived, just a voicemail if I was lucky. My Sprint signal showed about the same strength, but the phone would never stay on it, even though I was able to place and receive calls reliably when on Sprint. It wouldn't take advantage of a strong wifi signal unless I put the phone in airplane mode.

Project Fi had me submit bug reports, and in the end they agreed there was some sort of problem. They set it up so that when I'm at my house the phone won't switch to T Mobile. It seems to be staying on Sprint now, like it should, but I have to test it to see if it will still utilize T Mobile when away from home.

They didn't really address the issue of not automatically switching to wifi. I think they still have plenty of bugs to work out, but I'm willing to give them some time to do so, for now, until something better comes along. My real complaint is having to keep track of data usage. Coming from 15 years of Sprint and unlimited data, it sucks to have to monitor my usage. They need to offer an unlimited data plan.