From the NATO website, referring to both rebreather and hybrid rebreather equipment.
" ... the main disadvantage is that it requires a number of steps to make it operational during a critical part of flight. The system, currently the most popular used in North Sea, requires the human to physically perform up to six separate steps after the impact phase of a helicopter ditching to make it work. If these procedures are not carried out the system is rendered inoperable and the survivor could drown."