I haven’t noticed many subtitles on TV in the US. But I remember watching Trainspotting with a college flat mate. He had no idea what was being said; as a Scot, I had to translate for him.

Among my friends and family, only the really thick UK accents aren’t understood very well. Glaswegian, Scouse, etc. The more common (on TV) RP, upper class Edinburg, and Highland accents are mostly fine.