Credibility in the sense that they understand the concepts, regardless of whether they accept them, for a start. Most creationists don't understand even the basics of the theory they're criticising. They have no credibility (for example, Answers in Genesis where Theland gets his ideas is totally intellectually dishonest. Ditto the Islamic Creationist Adnan Oktar (better known as Harun Yahya) who famously printed a book decrying evolution featuring pictures of fishing lures that were labelled as real caddis flies)
Opposition to evolution is pretty much grounded in one of two things; 1) religious conviction or (but often *and*) 2) scientific ignorance.
I'm not quite sure what aspect of the recent climate change debacle you're referring to - do you mean the recent UEA kerfuffle (which didn't discredit climate change whatsoever, but did reveal academics talk to each other like children and that records management in universities is cack)? In the long term, it doesn't really matter if scientists recommend each other, does it? It could waste some time and money in the short turn, but if you make claims, other people are going to test them. If they're massively false, they'll be found out. If they're egregiously wrong the scientist will be discredited, otherwise you're simply talking about the progression of science.