From what I have seen on this thread, the disadvantages of a FFM are:
- Possible difficulty with gas switching
- Difficulty with the AAS in an OOA situation
- Difficulty with inflating the SMB
- Larger dead air space - problematic for rebreather divers
- Flooding in an OOA situation
There is larger dead air space, which could be problematic for all divers. In any FFM or helmet, there is a resulting increase in anatomical dead space (the helmet or FFM). This causes increased CO2 which could also cause an increase in respiration and RMV for even Open Circuit divers. Increased CO2 is not only a concern for CCR's but for every diver. CO2 is narcotic and also causes a number of issues for both OC and CCR. In a dive helmet, you can turn on the air train, and flush the hat occasionally, but this mechanism isn't available in FFM's, unless you pull the mask away from your face and purge the mask... but it's not the same.
My opinion on flooding in an OOA situation, I'd have to ask, what technical diver is having an OOA situation and why? There's really no excuse for OOA situations at the technical level, even if there's an explosive loss of gas, there should be alternatives and a technical diver should be able to reach the surface without having to do a CESA, as generally a technical diver would have a virtual overhead (decompression stop), and making a CESA would only result in getting (probably) seriously bent or maybe dead.
Difficulty with gas switch can be easily remedied with a switch block. This is no problem.
Inflating an SMB can easily be handled with an "inflator attachment" aka "doggy dick"... this is no problem.
Me personally, the reason I don't use a FFM unless I have to (for communications) is because - unless I'm using the Kirby Morgan M-48 mask, the breathing characteristics of the FFM's aren't as high performance as my Apeks regulators. If I'm diving to 200+ feet, I want my high quality regs, and also I don't want any increased CO2 levels (which are virtually unavoidable in an FFM)