I don't know about that. 9/10 you see excessive spinal compression and people bringing their low back off the bench. Either that or they don't go anywhere near deep enough.
BSS are a hard movement, but they are easy to learn and relatively self limiting. There really aren't many effective ways to cheat (other than skimping on rom). Personally they are the most taxing movement for me, but also the best for my legs.
Walking lunges can also be really brutal, and there is some more room for error with them, reverse lunges are probably safer in terms of less likely to do it wrong.
In terms of machines though a good hack squat machine is fucking awesome.