Semirhage put an entire continent (Seanchan) in turmoil. Graendal put one little nation (Arad Doman) and part of Almoth Plain into turmoil.

I do not know the exact numbers of military troops in Seanchan, but I am guessing there are about a million (if Seanchan is anything like Randland). Semirhage guaranteed that a million Seanchan will not be at the last battle because they are busy fighting among themselves.

Besides the turmoil Semirhage caused in Seanchan, she has effectively (to this point in the story) eliminated Amador, Altara, Tarabon, and Almoth Plain from fielding any soldiers to help Rand. The Seanchan takeover of these lands never would have happened without help and guidance from Semirhage. Time will tell if Fortuona eventually capitulates and comes to the light side's aid. My guess is that she will make the pact with Rand, but more fighting, costing more men, will ensue before the alliance is forged.

In my opinion Semirhage was far deadlier than Graendal.

I do not attribute Rand's insanity and almost destroying the world to Graendal. Rand's problems are four fold: his insanity from Lews Therin, the taint of Saidin, the effect Padan Fain had on him, and Rand's own bad decisions. Graendal had nothing to do with LTT, the taint, or Padan Fain. Arguably Graendal had nothing to do with Rand's decisions. If Graendal did have a role to play with Rand's decisions it was a minor role.

In the end, I definitely attribute Arad Doman's fall to Graendal. No doubt there. However, that is just one nation in Randland.