The problem of safe locomotion of legged and wheeled robots, when trying to avoid falling, tipping over or hitting obstacles, appears to be a problem of viability and not of Lyapunov stability. Theoretically speaking, viability and Model Predictive Control are unquestionably related, but both can quickly lead to untractable numerical problems. We present here a promising approach for the problem of avoiding to fall in the case of legged locomotion that elegantly solves this difficulty. We propose then a brief discussion about what makes this approach successful with respect to the approaches proposed for the other problems where viability is at stake. This paper should be considered therefore mostly as a prospective reflection on the general problem of safe robotic locomotion.