My brother's wife is from the Shetland islands. He'd always know when she was on the phone to her family, because she would drop straight into the dialect without realising it. The thing is, the Shetlanders know that their dialect is sufficiently different, so they learn TV English as well so they can make themselves understood to outsiders.

I was in Glasgow years ago for a conference. Wonderfully friendly locals, I didn't understand a word they said.

I wouldn't find them "pretty much incomprehensible", although there might be one or two variations in accent or words that we might trip over once or twice.

It would be much different experience from speaking with someone with a different German dialect, so I'd feel the same.

I had it happen to me with a taxi driver in Newcastle.

Obligatory Limmy: https://www.youtube.com/watch?v=0YfRbNipdOg

NSFW language, if you can understand it.