The issue with those rating sites is that they might be paid, just like ICO sites do paid reviews. You should look for user reviews from multiple sources to be sure. Also in this forum there is plenty of information regarding scams. They might be harder to find but it's better to spend some time reading instead of losing your money.
It does not have to be a rating site, there are forum like this that even when you search on Google, you will be directed to, so as to be able to get other people's opinion about a place to gamble or not to gamble. One way or the other, there must have been those who would have shared their experience in a way after being scammed so that others will know it is a pure red flag.
It is better to always stay safe than sorry and I do not feel it is a hard thing to find unless you just want to go with a site that others are not paying attention to which obviously would add no value additional to your gambling anyway.