Some accents are harder to understand than others. Once you get used to someone's accent then it is understood.

Sometimes. Accents can be hard to understand from other areas, particularly those with strong accents like Scottish or Welsh I find (or some areas of urban East England personally). Others might struggle with Southern American accents for example. Different regions have troubles with different accents I find. Usually it's only one or two words though, you can understand most of what they are saying just some pronunciation differences.

I don't really have problems with other British people, even Scots and Geordies. Of course sometimes I might come across a word or two I don't know, but I can often guess the meaning through context. When it comes to people around the world learning English, I sometimes have no idea what they are talking about. I don't even really know what your question is.