I don't have a problem understanding various UK accents, but I watched an episode of Rab C. Nesbitt, didn't get a word of what he said.
In fairness, some of the US Deep South accents are nearly indecipherable (Honey Boo Boo, anyone?).
I'm reminded of a slightly OT story:
I attended the University of Southern Mississippi, in Hattiesburg. One of my buddies went to renew his grant and was asked for proof of citizenship. He looked at the woman and said "Lady, can't you tell by my North Mississippi accent that I'm a citizen?"