squat machine
I'll chime in on this. Leg press and hack squats are good leg machines that simulate squats. Squats are regarded as the king of exercises yada yada. But, they are not for everyone. Biomechanics are unique. I've trained with people who were definitely not wimps, not injured, who had incredible athleticism and strength, but who had no business squating because of their unique shape. If you have a high center of gravity, inflexible lower back, inflexible ankles etc..., then squats may not be the best leg pressing exercise for you. Front squats or machines may be better choices. Also, as a bodybuilder you may have other training priorities, such as more quad development in which case front squats or leg presses might be better if you hip squat.
I offer this simply to keep our minds open to other training ideas for individual differences. Also, stability training is not so much a concern for bodybuilders.
I would say this though, if you do care about functional strength and don't squat and rely on machines, it is crucial to strengthen the lower back to avoid a leg/erector imbalance. If your legs can press 600lbs, but your lower back can only cary 200lbs, then something could seriously give in real world strength application. Remember in the real world your legs are only as strong as your back and your chest is only as strong as your triceps.