In the U.S. I perceive MVNOs to be a scam. Like Tracfone was OK in 2011 but somewhere afterwards it became normal to go around rural upstate NY and no service and go around NYC and no service and go to LA and no service.
This year I got a real phone with a real plan and I have coverage in many places now me I can finally see why people get excited about apps.
That's kind of a broad brush, Google Fi is great, especially the fact that they work mostly seamlessly for the same price in like every country, and they're an MVNO.
It proves that you can’t trust the claim that “it is the same network”, I mean it is the same network but you may find yourself riding the back of the bus and there is no independent way to check what service you are actually getting.
This year I got a real phone with a real plan and I have coverage in many places now me I can finally see why people get excited about apps.