IMO(and you vets correct me if i’m wrong) GM’s work you lower back as the primary while hitting your hammies as the secondary. While glute ham machine does the opposite. GHR’s hit your hammie primarily and low back somewhat.
I personally have noticed marked improvements in my squats since using GHR’s regularly in my routine.
I won’t argue with you cuz I don’t know the mechanics of it. But I get more low back action than hammy work with good mornings. As I said, In My Opinion. Everyone is different.
I’m no expert either. It probably has to do with how you do them. Different stances, different form, etc. probably puts more emphasis on lower back vs. hamstrings and vice versa.