The Army and Marines used a lot of helicopters in Afghanistan. It wasn't that the Army was afraid to use helicopters there, it's that the high altitude terrain makes helicopter operations difficult. Few US military helicopters were designed for high altitude operations, with the Chinook being the most capable. Unfortunately, they're also unsuited for air mobile assault operations.

As seen by that crash a couple days ago on Mt. Hood, operating a helicopter high in the mountains is dangerous business that requires special training. Every helicopter design has two critical performance parameters: HIGE and HOGE. HIGE is Hover in Ground Effect, the maximum altitude the helicopter can hover where ground effect helps. HOGE is Hover Out of Ground Effect, like what you encounter when attempting to land in mountainous terrain. HOGE is always less than HIGE.

The Chinook's design allows all of the engine power to be used to generate lift, unlike a conventional helicopter where a significant percentage of the power goes to operating the tail rotor. The Chinook is one of the fastest helicopters in operation, again due to the tandem rotor design where there's an advancing blade on each rotor at all times. Conventional helicopters are limited in top speed by the retreating blade which must be increased in angle of attack with each rotation to keep the helicopter from rolling to one side.

I don't have the HOGE and HIGE numbers on the Osprey but it has several characteristics in common with the Chinook. I suspect the Osprey will perform well at altitude. However, the V-22's hydraulic system is pressurized at something like 6000 PSI, making it vulnerable to combat damage. A hydraulic leak at that pressure is strong enough to cut through metal. That system is probably the V-22's weakest link. Like every other aircraft, the V-22 has a performance envelop where it can operate safely. Operating outside of that envelop can be deadly, as proven by that deadly crash a couple years ago where the pilot descended at too high a rate and killed 16 Marines. That wasn't a design flaw of the V-22, it was a case of either inadequate training or pilot error. A pilot can destroy any aircraft by exceeding the design limits. That's just the nature of aviation.