I have never seen anyone break a touchscreen with a fingernail. Those ladies should be ninjas.
I suppose you could crack a screen with a ball point pen but a stylus is made to work on a touchscreen. Therefore it should not be able to break one too easily.
Smartphone screens crack mostly from dropping or sitting on them when they are in your back pocket. The former I could see happening with a computer touchscreen monitor but with no more frequency than a non-touchscreen. The latter would require some large back pockets.

By the way, I call smartphones with cracked screens The Spiderman Edition.