The bottom line is that the Smith-machine squat does not pose any inherent dangers to either the knees or the spinal column provided it is performed correctly, and no one to my knowledge has ever proven otherwise either in theory or through clinical trials (though "expert" opinions abound); but I invite anyone to do so if they can. However if you do not adhere to proper form when performing this or ANY other exercise you will in all likelihood encounter problems. The Smith machine has long been a viable alternative for those who wish to perform squats and pressing movements at or near the upper limits of their abilities with safety and confidence at times when a capable spotter or a power rack is not available.