Gee...when I was in Scotland back in 2001 I had no trouble understanding any of the locals. Even took the rental car into the country side outside of Edinburgh and no issues.
That's the east coast, posh there they are. Glasgae is on the Clyde on the west coat, posh they are not. Govan is by the docks in Glasgow, they think that that the rest of Glasgow are too posh for their own good.
Edinburgh and surrounds (e.g. Fife) probably has the most intelligible English speaking accent anywhere in the world. Edinburghers typically pronounce ev-er-ry syllable crisply and clearly and are quite slow speaking, in a measured and deliberate sort of way.
Really posh Edinburgh folks have an accent that is barely distinguishable as being Scots, it's more 'BBC' than the BBC has been for years. Morningside typifies the really posh Edinburgh accent, and Maggie Smith in "The Prime of Miss Jean Brodie" does a perfect Morningside accent.
If you want incomprehensible, you want my mate Billy Smith from Orkney. There's nothing rough or ill-spoken about the Orkney accent, it's just damn near incomprehensible to outsiders and Billy in particular speaks at a mile-a-minute. For some reason I can understand Billy with ease, but most people can't, and I have literally had to act as a one-way translator on occasions.