The trouble is many lifters do not perform inclines in an effective manner. First of all, if the angle is over 30 degrees, more of the lifting is done by the delts, not by the chest. Another flaw many lifters practice when they incline is they raise their butt off the bench, in essence, making the incline press a flat press. This actually takes away the emphasis on the upper pecs, which were the targeted area. This is usually done to lift more weight or get more reps.
Declines are also popular.
They tend to stress the lower or sternal pec. The biggest execution flaw is that lifters may start out hitting the nipple area, but as the weights increase, the bar tends to hit closer and closer to the waist area, with many lifters doing a 'belly bounce'. This shortens the range of motion considerably. Perhaps this explains why some lifters claim to actually press more doing declines than with regular flat benches. Less distance means that more weight can be lifted.