I have heard of such services before (not this website specifically, but similar ones) and they seem like a good idea because, how can you be sure of the legality of the coins you receive otherwise?

But what makes me raise my eyebrow is the fact that these services are free. If they don't charge you for their work, then they must be making profit otherwise, probably with your data.

Someone with experience can shed some light on the safety of using these services?