I went to Newfoundland and I went to a bar one night and met a guy from a small town along the coast and I literally couldn’t understand a single thing he said. He was apparently speaking English but it may have been Ancient Greek for all I was able to make out. The only way we could communicate was via the bartender, who would interpret what he said and tell me. He had no trouble understanding me. It kinda blew my mind.
I am from a Canadian maritime province. I have had Americans (particularly from the south) who at least claimed they couldn't easily understand me, despite me understanding them just fine.