I always thought they were much safer than other knuckle strengthening methods, such as hitting makiwara post.
I used to bruise the hell out of my knuckles hitting one of those things (Makiwara post), and my hands would get skin up pretty bad hitting a 75lb vinly punching bag with no gloves on. That was about 8-9 years ago when I was about 13! (I know, I know, I was just a little idiot then...now I'm just an older idiot
Now the only type of actual knuckle conditioning I do is knuckle push ups on a hard floor.
I never experience pain while doing this exercise. I guess thats why I just assume them to be safer.