The MEL does not gauruntee it's safe, because it does not (can not) possibly cater for all the possible combinations of circumstances. Aircraft I've flown have usually had some legal weasil wording to that effect anyway.
May I dispute such interpretation? MEL is considered to be safe, and you even can carry SLF on such flight...if it so I will not fly in any aircraft that is MEL!
FSLF