If I remember correctly, Graendal remarks that Someryn is strong. I believe she tells Sammael she won't teach Someryn certain things, and I paraphrase "because I won't risk both strength and skill" in someone like Someryn. In other words, we already know the Forsaken consider themselves in a whole other league, in terms of skill. Graendal is admitting Someryn has great strength. And as we see in Forsaken, they NEVER want to admit someone is stronger than themselve. So, Graendal possibly discredits Someryn's superior strength by highlighting her lack of skill. It's the classic thing people do all the time. If someone is better than you at something (i.e. Someryn is stronger than Graendal) then you draw attention away from that by changing the subject (e.g. Someryn lacks the skill that Graendal has).

I think Someryn is stronger than Graendal. At the very least, my explanation above is possible. We do have some missing people on the list of 6. Why couldn't one of them be Someryn?