I have not rode most of them to determine if those mentioned were actually really bad, but I believe that it is not fair to ride one certain model of now-a-vintage bike, which differs greatly each other depending on how it has been treated, then decide whole model was all like the one you ride. Someone commented in Bike Exif's facebook page that 72 CB750 was one of the worsts. I own 71 CB750 and and it still keep up with more recent bikes at corners. So, I am sure that he got ill-maintained one, though handling was not the strongest field of CB750...Or maybe he is comparing to modern crotch rockets?
As for Guzzi, I read an article about a Motoguzzi fanatic, who is an expert on 60’, 70’s Guzzi, talking about how each bike were different. “variations ranges from subtle difference in paint colours to different parts on supposedly identical bikes.” So, I am sure that you can't just circle Guzzi as crappy bikes.
I gotta say I am surprised that people assume the bike’s handling was bad just because they wrecked it. I guess you are supposed to ride it like you stole it as soon as you get on the saddle? I thought you are supposed to go easy until you know bit about the bike’s ability. You don’t go attack corners till its limit right away, right?
I believe that most of bikes could be a great bike if it was ridden by great riders.
Well, again, you can’t make everyone happy. Someone always has to #$%*.