I don't think there is any doubt that Lanfear is stronger that Granedal but equally Graendal seems to be pretty handy with compulsion. Now when she took on Moggy & Cyndane she has an angreal and Cyndane was weaker that Lanfear but it was compulsion that was her weave of choice.

We know a few things about the forsaken. Even the braver ones will not throw away their lives without a very good reason. Secondly I beleive that once one is close enough in OP strenght any duel could go either way, and it depends on a lot of things like the surroundings, who sruck first, how prepared the other channlerer was, just plain bad luck etc. Say Lanfear attacked Moggy, she would probably win 70% of the time but the 30% is what keeps Moggy breathing because why risk it unless you are very certain of victory.

Lastly Granedal may not be as strong as Lanfear but her superior compulsion skills act as a force equlaliser. She is more skillful with this very useful weave so she does not have to be as strong. She gets more bang for her buck that say Lanfear. Like wise Semirhage knows a lot about the body and in particular the brain. She can probably use offensive weaves that are a sort of reverse healing. This makes her dangerous even to a stronger channeler.

One last thing RJ has said that the forsaken lie even to themselves hence Rahvin's belief that he could take Lanfear. Its probably a case that the forsaken who were always at the top of the pile are a lot more accurate in judging their abilities and that of their enemies hence they don't get carried away with overconfidence.