US Mobile has been pretty good. Cheap, lots of non-throttled data, works on top of all major carriers in the US (not at the same time unless you pay extra), and data speeds have not been an issue so far. Personally, I've found that my cell service has functioned better than when I used Google Fi (e.g. I previously had connectivity issues in trains). I'm guessing this is mainly because I use Verizon (Warp) as the underlying carrier on US Mobile instead of T-Mobile.
Other than basic security features like preventing your number from being ported out without your permission and so on, I don't really worry about security/privacy for the carrier since I just use a VPN (and Tor when necessary).