When looking at science not as some hypothetical ideal in the Philosophy of Science, but in terms of what is actually practiced, science is a domain where expert opinion matters a great deal and in the real world it plays a significant part in how scientific claims are weighed which is good since equally weighing all claims would make the entire enterprise hopeless. If pointing out that someone's not an expert while weighing in on a topic requiring expertise is an ad hominem, then ad hominems are actually a reliable and veridical form of reasoning. It's rational to protest a drunk stranger with no training taking the controls of a flying jet.