Strength IS a concern to them.

Graendal was concerned that she would not wish to confront Rand's power if Rahvin and Ishamael fell to him.

By all accounts, Rahvin was not as skilled as Graendal, but his sheer power made her consider him a dangerous enemy.

Jordan felt that Aginor could undoubtedly dispose of Balthamel if he so wished. Aginor has a weak personality, and is not very good in confrontational situations. So that alone says that his strength advantage over Balthamel must be signficant.

Moghedien never dared confront Lanfear, due to Lanfear's vastly superior STRENGTH.

Strength is a very important factor. Not the only one, but a very important one.

I honestly don't believe Semirhage would be so respected if she was barely above Moghedien in strength, as you suggest. And especially not if there was such a huge gap between her and Lanfear.

I think Semirhage was second only to Lanfear, and equal to Cyndane and Alivia. And that Graendal is of average female Forsaken strength. But Graendal's tactical advantage, is her Compulsion Talent. That makes her more dangerous than someone like Mesaana, who is on the same strength level as she is.