i've tried an Air2 on halcyon before and have got the following issues:
i. in an emergency air-sharing incident it will be hard to maintain good trim whilst ascending/swimming with a panicked buddy. it's likely to be in a vertical upright position - problem for swift currents for e.g.
air dumping when air in bcd is expanding is also a problem as u are already using the Air2.
ii. donating primary reg which is not that long makes the buddy swim close to u. entanglement of gear or just a panicked arm knocking off your mask will escalate the issue to even more panic.
iii. washing bcd with air2 by resort people who submerge the entire gear set might send water into the Air2 causing corrosion and ultimately lower the performance of an equipment u will rely on in an emergency.
iv. performance at depth (40m or more) is doubtful. additional expense when one's diving progresses into nitrox, trimix, cave, wreck etc.
v. still better to use long hose, backplate config. that is equally good for simple recreational diving and more advanced diving. one set fits all, no need to think which gear to pack or which hose to donate - all the same system and develops excellent muscle memory especially in critical situations instinctive actions might save you or buddy's life.