We learned a lot from their experience. Their Mi-8 and Mi-24 helicopters were very vulnerable to Stingers because the engine exhausts are right next to the transmission right below the main rotor. The Discovery Channel had a Wings episode several years ago on the Hind in Afghanistan. It showed some spectacular footage of Hinds being hit. The missile would impact right at the transmission and literally break the fuselage in half. It was a gruesome sight.

Most US military helicopters incorporate IR supression in their designs, making them much less vulnerable to heat seeking missiles like the Stinger. They're still vulnerable in a hover to simple unguided rockets like the RPG (rocket propelled grenade) used to such good effect in Vietnam and Somalia as shown in "Blackhawk Down".

While I don't know the specifics, I'd be surprised if the V-22 doesn't include IR supression. It does include driveshafts to crossfeed power from either engine to both rotors in the even of engine failure. However, those very high pressure hydraulic lines are a weak link. Personally, I'd hate to be in one going into a hot LZ. Then again, I wouldn't want to be in any helicopter going into a hot LZ, either.

As for the Mt. Hood crash, it's possible one of the engines lost power but it's too soon to tell. A helicopter is a collection of parts violently trying to rip itself apart, so any number of things could've gone wrong.