Camming Devices are the most complicated piece of gear to place well, if mastered they can offer solid protection in places that otherwise would be impossible to use. The reason being that cams can be used in parallel sided cracks.
One of the most important consideration in placing cams is that when loaded the device will produce a large force (about 3 to 4 times the load placed on them) that will try and force the placement apart, as such they may well lever off a superficial flake, so the intergrity of the rock is even more paramount then with any other piece of climbing equipment.
There are other important considerations when placing cams. Firstly how they are placed in the rock, the direction of pull should be along the shaft of the camming device. All the cams should also be in contact with the rock, and the cams should be somewhere between 25% and 75% between the maximum and minimum width the camming device can achieve.