Review website for research groups
There are unfortunately numerous issues that I see with this. 


Many research groups are small enough that anonymity would be next to impossible.
Similarly, most professors only advise a few students at a time and it would take forever to build up any sort of meaningful amount of reviews on any one professor.
Most research groups are transient. As students graduate, the group will be changing on what is likely a yearly basis. Just because Jim and Jill were great to work with in the Bonsti Lab three years ago does not mean that Jim and Jill will still be there now. 
RateMyProfessor is somewhat of a cesspool at times. It can certainly show overall trends (such as 23 reviews that mention that the professor cannot write a clear test or 19 reviews that all mention how messy a professor is), but such trends would be too subtle to notice over just a few students. 


One is likely best served by just speaking to current graduate students in the lab of one's interest. Students usually know if there is an overall troublesome situation in a given lab.