For myself, I'd have to say myth. Because I really screwed up my back one time doing front squats, and have never hurt it doing atf back squats. Although my back is fairly easy to screw up.
maybe so maybe not,,, but I would think that it would be hard to slump over on front squats like I see a lot of guys doing on regular ones. If you did your ass would be on the ground. This might save your back if you have sloppy form.
You can stay more upright with front squats, but I'd still say that is a myth. If you're doing any amount of weight, you really gotta flex your abs and keep your core tight for any kind of squat or your back will be in danger of injury.