I haven't found mine stressful to use. I find that my non-viewfinder eye focuses on the distant subject, and the viewfinder eye simply sees the projected frame floating around subject seen with the non-viewfinder eye -- I don't find my viewfinder eye stressing to focus in a different plane than the non-viewfinder eye.
For those not familiar with the Kontur, it's a viewfinder that, when you look through it, presents an entirely black "image" with framelines on the blackness in yellow or white. So you can't see a thing when you look through it with one eye. But when you keep both eyes open, your brain superimposes the framelines onto the what you see with your non-viewfinder eye (it's a 1:1 viewfinder).