Some British accents aren't understandable, not even for the British, Liverpool slang for example.
Once I spoke to a lady from Liverpool in my profession as a nurse, I couldn't understand a bloody word of it, thankfully her husband could and trenslate it into estuary English for me.
I think the British has the same problem as the Dutch do, sometimes you can't understand people from a nearby village because of their accent.
For what about the American accent, it's good to understand for foreigners, except the things Americans use for shorten the things, you have to live there to understand that.
Well, I think that's the problem with any language or accent.