It doesn't actually mention that it will allow you to transfer data at the maximum speed that the network conditions will currently support. When I had T-Mobile with the included free international roaming, I could connect to a 4G network and my phone would show 4G, but it would be 2G speeds since that's what T-Mobile's contracts allowed.

But assuming they actually can provide unlimited data at full 4G speeds, its still a niche product. It would be $300/month if you used it every day so you'd only buy a pass when you know you're going to need to transfer a large amount of data that day so you'll still need either a local SIM, T-Mobile, Google Fi, etc, or just rely on WiFi.