I have lived in Scotland for the last 30 years, so I have a lot of Scots songs in my repertoire, and many Scottish friends who speak in dialect (not just accent). Indeed, I was born in Scotland, but taken to England as a baby, so hence speak with an English accent. My mother was Scottish, so I was brought up hearing at least her accent (somewhere between Aberdeen and Greenock!)
As other posters have said, some are better at doing accents beyond "their own" than others, and even with my background, I do sometimes cringe at some attempts at singing in Scots by those who really haven't a clue. However there are some songs, e.g. Adam McNaughtan's "Yellow on the Broom", where there are a good number of dialect words, and would you really attempt to Anglicise such a song? NO!
On the other hand, I was brought up in Suffolk, and there are so few people who can actually do a Suffolk accent other than those brought up there: it all ends up as standard "BBC rural" with rolled rrrrrs (no, they don't do that in Suffolk, that's more West Country). In Suffolk you get the elongated vowels and diphthongs: e.g, a word like "there" becomes "theyere".
Have to say, I go with what our Hamronium hero has said; all makes sense to me!