I wouldn't say that all the MN coins on there are scams. I know of a few of the project/coins on there that are great, hard working teams with working products. But you're right there's a lot on that website which are plan money grabs, offering high ROI to lure people.
But like any investing you need to make sure you do your research. With MN, don't let the crazy high ROI be the only reason you invest into it.
And of course like any smart website here is their ass covering stuff at the bottom-
MNO is a masternode coin monitoring and stats service. MNO does not research or recommend any coin. Do your own research and invest at your own risk.
ROI changes often and is not the most important factor. Please consider Dev Team - Community - PURPOSE/Platform - Liquidity - Wallet when making masternode purchases.