Call roaming or data/text roaming? I'm on their latest plan ($75) and data/text works for "free" in a ton of countries. I've used it in Peru, Mexico, and Iceland so far.
I'm in Germany at the moment, and I'm using a European SIM card instead of my T-Mobile in order to get stuff done. Germany's 4G/LTE is notoriously spotty, but at least I can get 4G most of the time.
You can buy T-Mobile "data pass", but it's ridiculously expensive; the largest pass you can buy is a 500MB, for $50 (!).
The competition is equally bad, of course. AT&T charges $10 per day (!) for international roaming, meaning you're given the same amount of data as in the US.
[1] https://www.t-mobile.com/travel-abroad-with-simple-global
(LTE has a maxmimum of about 450 Mbit/s, but in practice it's closer to 15-50 Mbit/s down, maybe half that up.)
Maybe someone who knows mobile technology could chime in.
I used T-Mobile for a trip in the past and found it to be pretty frustrating.
One thing that I would add is that you have to really internalize what it means to have a severely rate limited connection. You have to control your sync settings. if you just landed and all your services are trying to sync, you'll be using all the bandwidth and everything wont work.
Uploading photos was a pain point though, and a major one: 2-5 minutes per image.