Google Fi is not deprioritized AFAIK and it's been discussed in the MVNO subreddit a bit.

That being said, most ARE and they are damn near unusable now in places that are growing.

Here in Orlando, Mint mobile can't even stream Spotify if you are stuck in traffic, that's how bad it has gotten. Forget being downtown or at an event of any kind.

I use Google fi specifically because its not lower priority.