By coincidence, I observed a possible example of this last night. I was rewatching the old "Doctor Phibes" movies with Vincent Price. Both films are (stylishly) directed by Robert Fuest, but Fuest had a hand in writing the script for the second movie as well--and, based on these films at least, he seems to have been a much better director than a screenwriter.
Both films are directed with a certain flair and visual imagination, but the script for the second movie is a choppy, clunky affair. The dialogue is notably less sharp and packed with endless expository monologues where the characters laboriously explain the plot to each other. Granted, the sequel was allegedly a rushed, somewhat troubled production that suffered from a heavy degree of studio interference and friction among the cast--but the two movies do seem to illustrate that letting the director handle the writing as well is not always a good idea.
Meanwhile, yes, I have seen scripts that have been worked on by at least twelve different writers . . . .