Most carriers offer unlimited data now that becomes deprioritized at a certain point. That means that if the network is congested, other customers are prioritized above them. However, this isn't deprioritization. This is a plan that is throttled down to 256kbps.
Before the return of unlimited, most carriers got rid of overage charges and just slowed phones down to 2G speeds.
Given that Google Fi is just T-Mobile service, this doesn't seem that interesting. First, it's not $30 for a single user. It's $30 per person for 3+ users. I could sign up for T-Mobile Essentials for $30 per person for 3 users and get 50GB of premium data and then just be deprioritized after that rather than slowed even if there isn't congestion.
If you need more than three lines, T-Mobile Essentials is actually cheaper. Again, you get 50GB of premium data and beyond that you're only deprioritized rather than throttled so most of the time you get a full experience.
This new Google Fi plan doesn't include international so it's not like it has that going for it.
Visible from Verizon offers $40 unlimited for a single line down to $25 for 4+ lines - taxes and fees included, and each line gets separate billing so you can share with friends/strangers. Google advises that taxes and fees will be 10-20% on their plans so you can expect that $30 to really be $33-36. Visible's data is lower priority on Verizon's network so things might be slower if there's congestion, but you don't get throttled harshly at 22GB like Google Fi.
There's a huge difference between getting deprioritized and getting 5-10Mbps during congestion and getting throttled down to 256kbps. One still feels like unlimited with the caveat that people paying more might be given some priority. The other simply isn't unlimited. It's a 22GB data plan where you don't get overages, you just get a nearly useless phone. Heck, with deprioritization, most of the time the network probably isn't that congested and you won't get any ill effects.
Without the international roaming, what's the point of Google Fi? It seems like it's just a more expensive plan that isn't unlimited.