That would not be dangerous, it would be stupid, like this comment. I don't think anyone was suggesting such silly extremes. The very idea that playing piano is "very dangerous" ... LOL, if that's your definition of dangerous you need to get out more... LOL 
Steve, where is your head?
Overpracticing anything (but
especially Dohnanyi exercises, and I'm assuming you've never heard of them) can lead to carpal tunnel and tendoniticis. And last I checked, you need your wrists and arms to play the piano. So if you're risking an injury that will literally prevent you from playing, then yes, Steve, that's pretty dangerous in my book. I understand you have a cute little vendetta against me for my "negative attitude," but grow up and face the facts for a second. Please.
From "Developing a Healthy Piano Technic":
What is "Piano Technic"?
THE STUDY OF MOTION
Piano technic is the STUDY OF MOTION - - - of trying to find the best ways to move to develop coordinate playing - - - NOT the study of how to develop muscles. My students' most common choice to the pre-quiz is usually "How to Develop Muscles" because everything else in their life boils down to building strength and endurance --- whether it's track, baseball, aerobics class, weightlifting, or even mental endurance. But at the piano, if you focus on finding the right ways to move, this in turn LEADS to precision and an effortless sort of strength.
Correct motion overcomes technical problems and produces virtuosity. Correct motion is based on the way that our bodies move naturally. It's the way that many child prodigies move instinctively, and it's the way that we can move so that we never feel pain or tiredness when we practice and play the piano. Most pianists tend to concentrate on trying to improve their playing by "developing stronger fingers" through exercises and lots of practice --- to develop that so-called "weak 4th finger" and pinky - - - to develop endurance and so forth. The problem with this mentality is that as soon as you FOCUS on the MUSCLES and on building strength and endurance, you become TIGHT and tense. So instead of focusing on building 'strong hand and arm muscles' and endurance through hours of technic exercises and scales, try to FOCUS ON FREEDOM OF MOVEMENT -- on making everything feel FREE and EFFORTLESS as if you're not having to use any muscles at all!
By focusing on finding the right ways to move, this in turn LEADS to precision and an "effortless" sort of strength. (See "Why I didn't actually need all those bulging muscles in the 1st place!!" below.)
"The Taubman method demystifies mistaken ideas of how to play the piano --- that muscle strength and constant repetition are not the key to success."
"With other teachers I practiced passages repeatedly and didn't understand why my back and arms became tired. Some teachers have a natural technique that works for them, but they have a hard time explaining it to their students. Often they teach the way THEY were taught [not the way they actually play]. At Julliard ... everyone did exercises, but there was no relationship between them and the music ... [the exercises were simply a waste of time]."
Piano exercises simply to "build muscles" are a waste of time. Some exercises are very dangerous. Piano technic exercises that involve extreme stretching and finger isolation, like Dohnanyi and Plaidy, can be very dangerous. Mrs. Taubman explains."If the arm is free and unrestrained, it naturally reacts with corresponding motions to the direction of the motion of the fingers." In order to have "active fingers and a quiet arm", ala Czerny, there would have to be a restriction on the arm's motions. Can you imagine the effect on the fingers freedom of action with a rigid arm dragging behind it? In fixating the arm while trying to move the fingers quickly, it could be compared to 5 racing horses pulling along a train without an engine. It is clearly expressed in Czerny's exercises recommending "active fingers and quiet arms". Exercises for creating powerful fingers, and stretching the fingers, are responsible for most of the injuries and limitations in musicians."