Im going to play devils advocate here and say that I use the smythe machine for most of my excercises. Everyone says stablizers, stablizers, stablizers, for every thread about the smythe machine or its unnatural. They make it seem like if you use the symthe your muscles will atropy and you will be falling all over the place when you stand or walk. Its not true at all, and I thought the exact same thing before I started using it. But I have been using the smythe machine for about a year and a half and its hasn't caused an injury and my legs have grown quite well.
If someone started squating 135lbs for 6 reps on the smythe, and 2 years later they are squating 600lbs for 6 reps their legs will be much bigger. The argument could be made that free squats will promote better gains, but if someone is free squating 600-700lbs I would hope they have some spotters on hand for safety reasons. I certainly wouldn't want to be doing any free weights without a spotter because I go for my pr every workout, which is why I use the smythe machine so much.