Whatever is going on in this thread aside, I’ve been thinking about this. A 405 squat in a Smith machine is “far” “easier” than a 405 barbell squat, so obviously the bollus of stress is not equivalent at the same load. Perhaps the increases when utilizing such a device peter out for a similar reason pressing movements stall first, i.e., insufficient stress to force an adaptation.
I’m sure many here will say “duh” to that. My contention is that because of the nature of the movement performed in such a fixture, you can never really do what you need to do to increase the stress if a strength adaptation is desired. If there’s insufficient stress, you could add more reps or sets, but that means lighter weights. You could add additional days, but that just means more “volume,” not weight on the bar. You could force more weight on the bar, but because you’re not eliciting a strength adaptation, you fail and then drop reps.
This is probably related to Santana’s Theorem that the fewer the number of joints the movement involves, the greater the number of reps are required to accomplish the relative same strength adaptation. The causative mechanism are the same, i.e., stabilizer muscles and the fine balance component.