jhine,
"What is it about their drawings that is "off ""
Their illustrations used to be childish and crude. I remember a friend from school requesting a specific booklet that had been advertised in a leaflet. When I gave him the booklet he laughed at the cover. This was in the early 70s - the booklet from the 60s.
Then they got a little better but when they illustrated crowds of people it looked like they had been cut and pasted. One had a bunch of people going through a mountain pass walking towards paradise. The people in it were gazing in different directions making it seem like they were oblivious to each other.
An example:
They have also been known to 'steal' other people's illustrations. Such as this slightly altered one: