Project Fi mostly uses Tmobile and Sprint (it switches to whichever signal is stronger) as well as wifi calling. Wifi calling works OK usually, though it can take longer to connect (ie a few seconds of “Hello? Hello?” when picking up).
Comparisons to Verizon depend a lot on your location. For instance Verizon here is better in rural areas, but worse at my workplace. YMMV.
Finally, Project Fi has great support. They are easy to get on the phone, and very willing to ship overnight replacement phones, SIM cards, etc.