As I recall, Fi piggybacks on T-Mobile's international roaming at 2G speeds (e.g. 128 kbps) except where "3" service is available. When connected to "3" a Fi phone has full speed. All data, domestic or roaming, is charged at Fi's standard data rates.
T-Mobile includes throttled roaming data at no additional cost. You can buy international passes for "3G speed" (512 kbps). Which is also on the ONE Plus upgrade.
We had Project Fi on one of our phones and it worked pretty well. It had domestic coverage in places T-Mobile didn't but I always had to check to make sure I was using available WiFi and had all the data saving features turned on to avoid high bills. We only left Fi to put that phone on the 55+ plan before the rate went up.
Google Fi almost always offers the fastest data speeds available while abroad. If a region has 4G LTE service, you’ll get LTE speeds.
With T-Mobile ONE, you will get 2G (128kbps), and with ONE Plus (+$15/line/mo), you will get 128 x 2 = 256kbps. To me, even 256kbps is not enough for IG, FB, applications with videos/pictures like CNN or navigation like Waze. I missed the big promotion from Google Fi since I was traveling abroad. I plan to switch to Google Fi next time when they have another big promotion.