Clausen scored 3 less points than Cutler did against Detroit this season. You'd have to clarify your definition of "success" on that one. He looked competent. That's about it.
McCown was great last year but it was clearly an outlier.
And Trestman's history suggests that he won't last more than 2 years in Tampa if that's where he ends up. Year 1 is always great, followed by a disastrous year 2. He probably deserves a job as a QB coach, but I wouldn't hire him as an OC.