In my opinion they can be strong enough, but they have to be well made. My first vibrator was a poorly made glass vibrator, it transferred vibration well enough but it broke within a day. Among other things that went wrong with it- one problem was that the only thing holding the vibrator in place was rubber bands glued onto the glass. The vibrator portion was removable so I removed it once, gently and already one of the rubber bands came off. Eventually the other rubber band snapped off so it became useless. I've steered clear of glass vibrators since, might try again some point, but I don't too much trust the icicles brand for that.