Many athletes and bodybuilders use smith machines when benching or squatting. These machines basically exaggerate your strength by taking over part of the lift for you. So, while they are excellent for the ego, they’re not letting you lift to your maximal capacity.
Let’s say you had an option between using a flat bench for 200pds, 6 reps or a smith machine bench for 250pds, 6 reps. Does it not remind you of assisted reps. A smith machine is like an assisted rep machine – by taking over parts of the lift, it takes stress off of you. The more stress taken off your muscles, the more you can lift. However, the more stress taken off you, the less your muscles are increasing. To keep it simple, use free weights.
In addition to the above, there are two more problems – decrease of ROM by 50% or more (depending on machine), and increased possibility of injury. A smith machine forces you into a fixed groove, which is not so ideal. If you watch two people squatting and using the same technique, due to differing builds, the paths the bodies pass through will differ. When you are forced into a fixed path, not only can it be less than advantageous for hypertrophy/strength progression, but it can also be potentially risky since your body may not be meant to travel that path with that quantity of weight.