I can see his point of view about it, though. Murray isn't playing the role the same way he would as in other films, although that is the point. He is supposed to be in a miserable mood, being the Scrooge of the film. Maybe Roger felt he should've lightened up a bit at times. Roger later interviewed Bill for Quick Change and Murray asked him to explain that review and Murray did admit that they made it very fast and there were problems with the director Richard Donner. He did love the script (well, it was written by his two buddies, so of course he would) but his version was different than what Donner saw. But he did tell Roger it wasn't that bad. I do know a few things: 1. Sydney Pollack was supposed to direct it originally, but the writer Michael O'Donoghue and he had disagreements. 2. Murray's speech at the end was total improv. Maybe that's why some people don't like it.
Siskel and Ebert use to occasionally admit they goofed on reviews. They use to do shows about movies they panned that they changed their opinion's on. Toy Story and Slapshot were two I remember off the top of my head.