Having been a juror, I for the most part do not approve. The question is not whether Bill Cosby is a sleazy rapist who has drugged women and then raped them. The question is whether Bill Cosby raped a particular woman on a particular occasion. Prove the charge with evidence about the (alleged) crime. If the defense claims character, then his past is relevant. Otherwise I don’t think it is.
Yes, admitting testimony of other unproven allegations is prejudicial and opens the whole case up to being overturned on appeal.
If they can’t prove this charge, then testimony about other charges they also can’t prove demonstrates nothing.