I think that it probably has the same stresses as cycling or hiking, depending on how much climbing and descending you do. My knees never hurt when I ski.
Just to warn you, Catrin, there is "downhill" in x country skiing. Of course, it's minimal on beginner trails. But when I started, over 20 years ago, I never got off of the beginner trails, and it was boring. After I started cycling, I became more adventurous and it wasn't really a big deal. The climbing is like climbing in cycling; you can do it slowly, it can be challenging, but it's a real accomplishment when you are done. I am much more adventurous with descending on skinny skis as opposed to on my bike. Again, I am not a speed demon, but I've done some tough stuff.
Can you tell that I love this sport?