I think slagging him off is the wrong term.What people are saying is that THEY didn't think he was good enough at the top level.I think most people know he did ok at the level below with all the attempts he had,but slagging him off? They are not.But they don't keep bringing it up on this message board.
Every few months you can guarantee that someone will try and belittle Warnock though. (And that's being charitable about the rubbish that is spoken about him!)
In the last 39 years we have only had one other manager that has brought any success comparable to that which Warnock brought. And incredibly, there are people using this forum who slag him off as well!