There's an old helicopter pilot saying that if something isn't going wrong, it's about to.

Vertical flight, either in a helicopter, Harrier, or V-22, is an unforgiving environment. The Harrier has a much higher accident rate that other fighters. Any loss of engine power during a hover is a disaster and the pilot has little time to eject.

Vertical flight aircraft have more complicated control systems than most conventional aircraft. That means there's more critical stuff to go wrong, usually at the worst possible time. For example, the V-22 has drive shafts to crossfeed power from one engine to another so both rotors are powered even if one engine fails. No other propellor aircraft does this. It has to use a combination of differential rotor tilt and other things to control the aircraft in all three axis during slow flight. In a similar vein, the Harrier uses a puffer-jet system for attitude control during hover. Ordinary aircraft don't require such complications. Furthermore, hovering flight requires a lot of power so all of the components are under a lot of stress.

Helicopters use a complicated rotor head assembly that includes complications like having to increase the angle of attack on the retreating blade and decreasing the angle on the advancing blade during forward flight. Without this system, a single rotor helicopter would have a strong tendency to roll towards the retreating blade side because of the different airspeed between advancing and retreating blades. Helicopters also have things like transmissions and in most cases tail rotors that are essential to safe flight. More stuff, more stuff to break.

Finally, vertical flight aircraft have operational limits that come into play in countries like Afghanistan. For example, every helicopter has two critical performance parameters: HIGE and HOGE. HIGE stands for "Hover In Ground Effect", while HOGE stands for "Hover Out of Ground Effect". These are effectively the operational ceilings for the helicopter. In every case that I've seen, a helicopter's HOGE is lower than the HIGE. This becomes real important as the pilot attempts to land.

Afghanistan is a mountainous country with some very high elevations. Many helicopters are at or near their operational limits in that country. Combine that with high winds, sand storms (blinding and corrosive, a wonderful combination), blizzards, etc. and you have a tough place to fly helicopters. Remember, helicopters don't have the luxury of cruising along at high altitude. To do their mission, they have to get down and personal with the terrain.