That being said, I wouldn't call them dangerous, just "un-ideal". It seems to me those devices are a poor solution to a non-existent "problem". Sure they eliminate a hose (which some people are just obsessed with), but they then require you to use a longer corrugated hose which in my opinion is more obnoxious than having an octo hose. If you're going to use one just be ready for the extra challenge of maintaining your buoyancy while you're sharing your gas.
To me "ideal" means having options, especially in an emergency. In an OOG, using the AIR2 requires you to make a face to face, vertical, direct ascent to the surface. In addition to the donor breathing off the device that controls their buoyancy, having two divers, face to face and vertical, will present extra challenges for controlling buoyancy.
This configuration completely eliminates the option of making a swimming exit as well as a direct ascent where both divers can be face to face and horizontal (much easier to control buoyancy).
While it's not dangerous (if practiced), it is limiting as far as what options you have in an emergency.
I do think it's dangerous to use if you AND your buddy haven't practiced with it. In other words, I definitely don't think someone who dives with "insta buddies" should consider it an option.