"Cue one ton of bricks from Jim"
Nope - quite agree with you on some of his earliest recordings.
The song you are referring to is Jock Hawk's Adventures in Glasgow, and there, it is not so much the accent that he has difficulty with but a rather heavy-handed attempt to imitate the 'plooman's' style of singing.
Having listened my way through the recordings of the Critics Group meetings recently, I came to the conclusion that he made a god-awful job of a Liverpool accent (my own).
MacColl's approach to accents was never, in my opinion, accuracy, but an actors one of producing a generally neutral Scots accent in order to keep the songs accessible but to leave them as Scots songs.
I remember seeing Matt McGinn at the Edinburgh Festival in the sixties playing the gatekeeper in MacBeath - didn't understand a word (and my dad was born in Glasgow).
Jim Carroll