It takes an exceptional actor to do a really good American dialect. There is a “General American” dialect that news and radio hosts and announcers practice, but it is largely nonexistent in nature. :) Most accents are regional. Even if the actors get dialect right, usually their resonance/placement is a bit off and that’s a red flag for me. It’s a bit hard to describe, but when you study accents and dialects, you don’t start with the vowel sounds but you start with what drives the differences between dialects (in English at least), which is where the speaker focuses the placement of their sounds. It’s a bit difficult to describe in text, but imagine that much American English comes from a place more forward in the mouth than British English, and Australian English is a bit like Cockney - a mix of forward and rear placement - but with more nasality. Highland Scottish is way in the back of the throat, but the lowlands are further forward.

For example, I felt that something was off with Hugh Laurie’s dialect when I watched House and it was his placement; but he tended to cover it and distract from it with his character acting, being a bit growly and grumpy. Still, to a native’s ears something didn’t just sound right.

The worst is when British actors attempt to do a Southern American dialect, it’s just so distracting, I’m thinking “where is this actor really from” instead of enjoying the performance.

I feel that most Americans, even when not specifically trained in dialects, will still get at least some feeling that something isn’t right. The awareness may not even be conscious but it will definitely detract from the experience.