That's cause Talzin's skills don't really reside in direct combat. She's immensely powerful, more so on Dathomir, but in close quarters she's pretty low-tier. Nightsisters don't hold the candle to Dooku as we know; even with Dooku attacked at night and drugged they still failed.
Fighting here, on an open ground, with Dooku far from being suddenly woke up, wouldn't go well for them. Well, I guess Talzin+Ventress+5 Nightsisters would seal the deal, but if Dooku arrived with some kind of backup, a bunch of droids for instance, he could send them as cannon fodder and escape pretty easily.
And if he did, it would be game over for Talzin and the Nightsisters. We would have the "Massacre" episode once again, only with Dooku in charge and a far worse slaughter. They didn't want to fight Dooku and Separatists for a reason.