Especially considering that so many of the main characters are walking around using angreals and sa'angreals, that complicate the picture even in such cases where sheer strength in the power decides the encounter. But honestly, it usually doesn't. If you look at the list of the Forsaken killed, captured or defeated, how many cases were there in which the strength in the power clinched the deal? There's Nynaeve's victory over Moghedien in Tanchico, but not many others. And in a rather different but related context, remember Lan's remark after his climactic fight in New Spring: "He was better, but he thought I was finished with one arm." The other guy was the better swordfighter, but he still lost.

Of course for Aes Sedai strength of power determines the hierarchy, which is fairly silly for precisely that reason, but not so among the Forsaken. Semirhage is feared more for her cruelty, ruthlessness and particular skills than for her general strength in the power. That she's stronger than almost all Aes Sedai and at least some female Forsaken, while weaker than Lanfear was, is clear; I don't really see the point of a ranking that goes much further than that.