Bruce Lee wrecked his back doing good mornings.
A lot of people have.
I do them all the time. I would never do them with locked knees with weight, that's a good way to end up like Bruce.
If you add weight, bend the knees and lean at your hips. Good mornings are supposed to work you hams too. Keep your legs steady and focus on just using your hams/back to move the weight, if you are using the quads you are doing something off.
With locked knees it's more of a stretch, which is good too.
If you have lower back problems and haven't seen a Doc, try using an inversion table for 30 minutes a day. You most likely have a disc problem, like i do.