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I need to correct this...
Manual (mCCR) and Hybrid (hCCR) Units have a constant mass flow valve...
You first need to ask the correct questions.
You could say a rebreather is a magical device that divers with high breathing rates desire... this being the Magic Carpet Ride side of rebreathers.
Unfortunately, unlike scuba, the rebreathers have a great many ways they can kill a diver, so you might say that is the "cost" of all this "magic".
Personally, I would use a rebreather only for very "mission specific" purposes, where scuba tanks just won't work....This is such a tiny percentage of all dives you could do, I see it as unlikely for most divers. Four miles back into a deep cave....a dive in open ocean with currents to an occulina reef at 450 feet deep for a 30 minute video session, with 4 mph drift current so no tanks can be used beyond what the diver carries.......there are a few scenarios I can imagine myself wanting the magic carpet ride, but not many. And these would be extremely dangerous, by recreational or technical use standards.
............... without as many risks as a full CCR. .