This is my experience with ONLY scratch grains as feed.
The previous owner of our new Roo and young hen fed them just scratch grains, besides table scraps. The hen was underweight her keel (breast bone) was very prominent. He was underweight but not as bad both had weird runny poo. Both treated for worms and mites/ lice.
We feed layena pellets and during the quarentine time both gained weight, he became more peppy crowing to announce his presence to our hens. The hen had laying problems with her first egg was a strange partially hard shell, when she was finally able to expell the egg there was a lot of bright red blood with it. I helped her with calcium, warm bath and massage to get the egg out. There was a loud "pop" when the egg came out. I then worked on her to stop bleeding but as it was internal I couldn't do much. I thought she was going to be ok as she was back to her loud annoying self but 4 hours later she died from internal bleeding. The egg had ruptured a portion of her reproductive tract about 2 inches inside.
I figured it was from not being given proper feed to help with egg production, she went insane on the oyster shell during her quarentine. Or she just had a problem in her egg formation system that would have killed her anyway. She was barely enough for a pot of soup.
I will not ever just feed chickens scratch grains, the breeding and manipulation that has been done to them no longer allows them to eat a diet that way. As an occasional treat to make them stir up bedding YES as only feed NO.