When you say you "never had a serious problem understanding people", do you mean you could understand them when you overheard them speaking to each other? Because that, of course, is the real test of how intelligible their language is to you. They may well speak a bit differently when speaking to an outsider. Also, you may be particularly skillful at understanding spoken English. I feel I have got better at understanding British dialects as I got older and gained experience of them. I was terribly confused by some dialects as a child.
With compulsory education almost everyone today has some knowledge of a standard language besides whatever dialects they have learnt. If you want to find someone who only speaks dialect X of language Y you might have to look in places where Y is neither official nor widely taught, or among very old people who never went to school.
I have heard Singaporeans, Indians and others speaking to each other and had no difficulty understanding them.
A problem I have come across in South Asia is people mixing languages. At that point are the speaking English? It can be quite disconcerting when someone changes language mid-conversation.
I have found Geordies and some Scots difficult to understand, but I think "mutually unintelligible" over stating it. There are reasons why Scots is sometimes classified as a different language. Other than that I have had difficulty with any form of British English.
Which? I have travelled to, worked in, or lived in multiple countries with English speaking populations (by which I mean some people speak English as a first language, though it may be a small minority) on five continents and never had a serious problem understanding people. Some slang and idiom and loan words, but thing much.