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The main ones are the ability to use coms and full face coverage in cold water.
A minor one would be not drowning in the event of ox tox, but that is only an issue during in water re-compression or if you have already really screwed up. Another con is the increased volume and the resulting excess buoyancy or the need for counter weights to keep the mask neutral.
The cons are the need (with most designs) to have a back up mask along in the event of a gas switch or an OOA gas share, and increased gas consumption (from using the com capability, or from a less than perfect face seal on a positive pressure mask).
It's a tool, but one you really only want to have along to meet a specific purpose.
I have dove the AGA and the ocean reef masks. They are cool but I don't like the nose blocks for equalizing. I'd love to get my hands on the KMB 48. Oceanic used to have a similar version they called the JMC super mask. Almost snagged the last one they had on stock a couple years ago. Then something else came up and it was pushed to the back burner. There are some others out there that do not have all the bells and whistles. They are basically just a silicone face mask with eyes that you put any second stage in. Never tried one of those.
Cressi Full Face Rubber Mask
and I see scubapro has this one as well:
Scubapro Full Face Mask, All Black
Problem with both of these is that like the AGA and Ocean Reef in an OOA you need to dump the whole mask. The KMB you just ditch the pod and go to any second or another pod.