Interesting!
I've only been to Mexico and Brazil since getting T-Mobile, but in both those cases I didn't have any speed issues with whatever they capped me at.
That said, I just looked and I can buy a 1GB international high speed "Data Pass" for $20, valid for 10 days. I didn't need one for any of my trips, but if that experience is atypical, I could see where Fi would be more attractive.
You can buy packages for 4g, but they are ludicrously expensive. As in, so expensive they won’t even show you the prices until you are a customer. If I remember, it was in the order of something like $10 for 100mb
There is definitely a data usage threshold where Fi pricing is worse, but if you spend significant time abroad it is hard to beat.