IMHO, OC-bail-out.
It is dangerous because (in random order):
- Lack of practice: It is hardly ever done *completely* from bottom to surface (including deco), except may be in the controlled environment of a course.
- Inadequate: Most people (that I know) underestimate the amount of OC gas required in a REAL OC-bail-out scenario.
- Ability: Depending on the individual configuration, it could be very fast to switch via BOV or having to fight the breathing urge (CO2 break-thru, over breath unit) to swap DSV to OC reg.
- Mindset: A lot of people might try to stay on the unit longer than necessary when dealing with problematic issues - which could be fatal if not bailing out in time. This could be from the mindset of staying on the loop mentality at all cost, and/or lack of adequate OC-bail-out...
I might be stating the obvious here... All dive gear (CC, OC) WILL fail at some point. The more one dives, the more likelihood one will face a failure. The more serious the dive, the more likelihood that the failure will be fatal. So continue to practice all bail-out skills (self or buddy) as much as possible so when the time comes, you could do it calmly.
Again, just IMHO.