So true. Even within the same exchange program, results can widely vary.
My high school used the module system, so the exchange abroad was scheduled for only 1 mod, ~1 month, but it was the last mod of the year, so we all got to stay on until school let out for the summer. The school we were exchanged with was a whacky boarding school - no host families, 0 academic expectations of their foreign visitors, and very hands off dorm parents. I increased my language skills by auditing classes, and made friends. Some of my compatriots spent more time in bars than in class. Maybe they’re fluent when drunk.
Thank G** things have improved since the 80s.