I don't really experience with cataracts, but limited or non-existent vision in horses and how they cope is generally more to do with personality and trust they have for their handlers.
I have a one-eyed horse that does just fine. I know a gal that has an Appy that is pretty much blind with "moonblindness" and he has a pasturemate that he clings to, but he rides fine with the owner. He trusts her that much. It just puts more responsibility on the rider to pay attention to the footing and obstacles.
A client of mine had a blind horse she couldn't deal with and gave him to some people that trail ride on him now. They love him and he works nicely for them. The only real concession they make for him is to always put him in a safely fenced area, and they put gravel right around the fence line inside the pen, so when he hits this gravel, he knows the fence is soon to follow, and that's the only pen they keep him in.
Personally, I would consider the horse individually and what I'm prepared to do to keep her before I wrote her off for vision problems. If she's flighty because of it, then I'd pass, but if she's laid back, it's worth considering.