And what happens with those who chosed Iorveth's path in TW2? There is no option for this. Nilfgaard could be less racist than Redania but, in the end, they don't want the scoia'tel anymore.
I think this would probably come down to the reasons WHY you supported Iorveth. It's the "ideology" path in TW2, but the game gives you two options - you can support Iorveth because you believe in Saskia's dream for a free state for non-humans, or because you want to help the dwarves protect their land against an invading army.
Personal view: If you picked the first reason, then Nilfgaard is probably the better option. They're not perfect, but they definitely don't seem as racist as the Temerians and Redanians. If you picked the second reason, then Djikstra makes more sense, as he's the one saving the North against an aggressive neighbour. Either way, Radovid would be an option only if you hadn't completed Now or Never - a pro-Iorveth Geralt isn't likely to support a king who is already carrying out a pogrom against non-humans.
To be honest though, as it's still going to be a snap decision, even without the violence, I don't think he'd necessarily analyse it this much. So unless you see Geralt as having extremely strong ideological views on the matter, it wouldn't be a major factor.
Also, it is really a good option to side with Nilfgaard if you chose Roche's path? It was all about keeping Temeria as an independent kingdom and a vassal state doesn't sound like an option for Roche. In the end, he is a patriot.
In TW2, he was willing to hand Anais over to Radovid and put on Redanian armour, you need to talk him out of it. He seems to be fairly pragmatic, and to take a long view. So no, I don't think he'd be happy in the long-term with Temeria being a vassal state, but I do accept that he'd take this route as a short-term solution.
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So what proof is there that Nilfgaard doesn't turn a blind eye on slavery, genocide and other such acts? Wasn't there a quest from a mother looking for her son that deserted where he couldn't take the baseless slaughter of innocents anymore? It's been months since I read some of the books but I don't ever recall where servitude, slavery, genocide were strongly punished by Nilfgaard.
I tend not to see this as Nilfgaard being inherently better (except on the racism against non-humans), but just the fact that, for the common people, the important thing is to have peace and the restoration of order. Who sits on the throne, and the colour of the uniforms and flags, is less important than the ability to farm and trade without some marauding army or bandits destroying everything.