True enough, but there are plenty of things that drop armor by half, some then apply AP to it, as well. Granted, these are pretty exotic, but if you are having trouble with an exotic yourself, then by all means use the exotic stuff.
For less exotic, elemental effects (fire, electicity, acid, cold) drops armor by half, including hardened. Chemicals still affect the possessed, the body doesn't become immune to them, and hardened armor doesn't help for that at all. The spirit might be immune to sarin gas, or to nanotech, but the meat body isn't.
The long and the short of it is this: possession mages are potent (very much so). They might need some tweaking even, rules wise. But they aren't invulnerable to mundane attacks, just really resistant. I have yet to run into anything that good old fashioned bullets don't take care of.
Also, I would like to know if anyone can find a specific page that states that spirits are immune to stun damage. I'm having a hard time finding it. Hardnened armor doesn't grant immunity. In fact, the only thing I have found that says anything about being immune to stun is for vehicles. Anyone else find anything on it? (Hard evidence please, like page numbers and paragraph headers.) This would make them vulnerable to a slew of new possibilities.
EDIT: Found an entry in Street Magic that explicitly talks about spirits taking stun damage. So, stunning a spirit is possible. Electrical attacks are looking more and more potent (Stick'n'Shock 4tw).