I prefer reading their platform which is written in their white paper rather than just believing on the ratings because they can still become a scam.
You are right, a high rating is not a guarantee that the ICO is not a scam. For example hashcard, initially has a high rating, but after the project runs some times and there are reports that the project is a scam, then ico rating down. Sometimes developers pay some funds to rate service providers to promote their projects with specials.