And the few times an instructor or operater has tried to use one in their defence has been used against them by the Courts.
My understanding (it is always difficult to know for a non lawyer what precisely the law says as report on applications tend to be done by parties having a vested interest and hiding relevant facts which would weaken their position) is that in general liability waivers in France are in the same situation, not considered as valid and potentially considered by the legal system as a hint that the party asking the other to waive their right knows beforehand that they are doing something they shouldn't done or avoiding to take precautions that they should take.