I understood Syannas' motives, but at the end I feel basically no sympathy for her. Olgierd von Everec did terrible things, but when I saved him he was grateful, he repented and he swore to become a better man. The good HoS ending fully satisfied me. But Syanna?
In my opinion Syanna should have been either more sympathetic or more competent. Ok, they treated her bad and I understand she wants to take revenge. But she failed, because she was not careful enough and consequently many innocent people from Toussaint died. Dettlaff and Anna-Henrietta are responsible for what happened as well, but Syanna has at least partial responsibility. She wants Geralt to understand her motives, but the topic of her failure remains to large extent untouched and as far as I know there is no scene dedicated to her that would show that she really cares about the innocent victims of her coup. To get good ending we have to feel sorry for her, but we rather not talk much about the casualties of the Night of Long Fangs, no, it's all about Syanna and her complexes. Maybe Syanna feels sorry for the casualties, but CDP didn't emphasize that.
If Syanna was not supposed to repent and revenge should have remained her top priority at any cost, she should have been more competent. I agree with hedop that she has basically no plan. She is described as manipulative, but in the end she is not trying to survive somehow, she does not present her point of view in order to be perceived as victim during the trial, she does not demand trial by combat. No, she just commits suicide attack, because she has no plan, no more ambitions, no self-preservation.
To sum it up, we are left with two villains that are better than Eredin, but definitely not as good as Gaunter or Olgierd. I was more impressed by Orianna than by both main BW antagonists together despite her having much less spotlight.
I am a little disappointed that we can decide about Syanna's fate only indirectly. Something like an unforgettable epilogue of TW2 - conversation with Letho, vodka and then either fight or farewell - would be nice.