>they piggyback off the T-Mobile network while Google Fi dynamically uses the best of 3 networks
This happens only in theory, not practice with Fi. As a Fi launch customer that stuck with Fi through late 2017, Sprint (and later US Cellular) service was always terrible and frequently unusable, no matter what metro area I used it in. My experience improved dramatically when I just kept my Nexus device around as a spare, and moved my primary SIM to an iPhone, where it was permanently locked to using TMO.
This happens only in theory, not practice with Fi. As a Fi launch customer that stuck with Fi through late 2017, Sprint (and later US Cellular) service was always terrible and frequently unusable, no matter what metro area I used it in. My experience improved dramatically when I just kept my Nexus device around as a spare, and moved my primary SIM to an iPhone, where it was permanently locked to using TMO.