I once did a "snap shot" with a Winchester Model 94 in .30-30 equipped with a tang sight. I didn't get the rifle seated to my shoulder well, so recoil was much greater than normal.
Let's just say that I'm glad I was wearing glasses at the time, as otherwise the aperture, instead of whacking the lens and scratching it, likely would have whacked me in the eye itself.
I think tang sights are fine for mildly recoiling rifles. Anything else, though, and it's just too close to the eye for comfort.