The angle on most of these machines can cause injury due to the awkward position of the hips and spine. It is very difficult not to push the lower back into the backrest in this exercise. Doing so places stress on the disc when loaded. Additionally, straightening out the torso (as in a squat) gets full recruitment of the hamstring and butt muscles. The leg press keeps your upper body fixed, which takes this out.

Better Alternative: Squats (barbell or dumbbell)