I went 6 weeks to Cali and Flo, and Americans are extremely sweet people more, much more relaxed and less speed than parisians (not hard).
If they are easy to have as really strong friends, that idk... That's different than being very sweet
From all my strong friends I kept from my time in SouthcKorea, none of them were Americans (there are a lot of Americans there).
But more north europeans, japanese that are precious as gold, thaï too.
I would say, yes, they are very friendly, but they won't be true friends that easily in my experience