u dont recruit your stabilizers as much when on the smith machine.
less muscle involved, not as compound as before.
tho ive read somewhere if u were training for quads exclusively, with the right footing, it is better than freeweight as u can focus on your quad without worrying about stabilizing.

yeah. dont understand why there is no squat rack in some gym.
if u dont mind restarting your squat abit, do it like me, learn to power clean and front squat that bar.
disadvantage is, i squat less as powercleaning even 1 1/2 of my bodyweight already makes me wanna puke, squating it for 5 rep is another story.
do these all over for 5 set is...
but the satisfaction, the ego boost ull get from people mirin your form. ah...