Well they get different "amps" there. If a place is strong in the DS, Sidious gets a boost from that.
The Nighsisters get their power from their connection Dathomir (at least according to Maul) so they will be far more powerful there then anywhere else.
I know I used to argue this vehemently a while ago myself, but canon *does* take priority over Legends.
I would say she's subject to change. In a sterile ship hangar in The Wrath of Darth Maul she was pretty weak, Sidious would've eaten her alive. She grows in strength depending on the plant-life in Legends, and in both Canon and Legends she has some special connection to the nature and/or spirits of Dathomir.
I heard from Filoni in some older podcasts that Talzin isn't *quite* alive during TCW, in the conventional sense. And we also know that she can give part of her physical form away for other purposes; to heal Maul, for example. But in doing that she limits her connection to the physical realm, and in the case of Maul, quite greatly. That's why she needed to siphon Dooku.
So she's capable of interacting with spirits and touching their realm while also retaining connection to the physical, which is pretty cool.
But this all makes her hard to use in debates, aside from the fact at her best she can give Sidious a real run for his money.
__________________ “The galaxy must experience the pain of death and the rapture of rebirth as I have. I will bring chaos. It is time for war.”
Her magick allows her to stand up against Palpatine's power, but it is magick, not the Force, so its almost like she's on a different frequency on the electromagnetism spectrum