I started incorporating the barbell hack squat into my training routine (a la beginner using 5/3/1) as an assistance exercise for the quads. I've done them on two occasions now. I know it's not a very popular movement, andit kind of feels awkward.
But I'm sure that any compound quad-training (which isn't a front squat - seems like that's the only compound quad training for a lot of ppl out there) is a good bonus.
They do strain my back more than deadlifts and squats.
I think this is because the bar is behind me, and when fatigue and weights combine I might have been jerking the weight instead of pushing my quads. Though bio-mechanically, it almost seems impossible to push that weight without moving the back? Or am I being an idiot? Mind you, there was just 50kg on the bar! I know, I'm a beginner, I suck, etc.. but.. 50kg = 35%ish of my deadlift? I couldn't even do a proper 5x10 with it.