Someone has argued the path is unnatural and the machine prevents the body from determining its groove. I assume they feel linear or lever machines are not as effective or not as safe as free weights. Although this argument warrants consideration, I am not aware of any scientific or empirical evidence to support this claim. Free weights certainly offer more variety of exercises than machines, but why restrict yourself to just free weights, particularly since greater progress can be obtained from exercises you are not accustom. See Restimulating Progress by Changing Exercises .
Machines seem to be as safe as free weights.
Even, physical therapists commonly use linear machines for rehabilitation. One closed chain exercise used for knee rehabilitation is the Sled Lying Leg Press . The smith squat can be adapted to accommodate individuals that may feel pain on the barbell squat. A center of gravity does not need to be maintained between the forefoot and heel since the machine can prevent you from falling over. Using a similar method we used earlier to examine the barbell squat , we can compare the torque forces of the smith squat variations of the squat.