Alumni respond with perspective. Low response rate. Hard to do. Delayed feedback to teacher. Tends to reward memorable presentations rather than straightforward yet valuable ones.
Alumni are evaluating events in their distant pasts, so comments are likely to be colored and vague.
Even more so than current students, alumni tend to evaluate subject matter rather than teaching. Peer evaluation by faculty. In this scheme each evaluator sits in on a few class meetings given by the professor under evaluation. Evaluators understand the difficulties involved in teaching and hence are less likely to be swayed by superficial gloss. Feedback to teacher can be immediate and excellent.