The show hasn't done a good job of showing us that there are all these other Northern lords and that nobody really likes the Boltons.
The problem here is that Ramsay doesn't really play by anyone's rulebook. He can go on and on about how "Reek" is more valuable to them than Theon, but that's just a justification that he came up with after the flaying, castration, and it's one that doesn't make a whole lot of sense on close examination. Roose knows that. The way that was played out, it seemed that he just was like, okay that bridge is burnt, moving on here.
Now the funny thing is that if Roose had been without a wife and Sansa was married off to him, I think she'd be in a much better position--at least in terms of not coming to physical harm. But she's not marrying Roose. She's marrying Ramsay, who is not that bright, or self-controlled, or possessed of a lot of game playing skills. He's a thug and a bully and shrewd rather than intelligent.
He's of the Joffrey variety, perhaps a bit smarter, but there are grapefruit that were smarter than Joffrey.
She is not safe. And the Boltons can do all manner of things to her. Also, once she produces a child, her value diminishes--quite rapidly. Same as it would have with Tyrion Lannister.