They definitely know about disease and what causes them, and what has hurt their patients, and my orthopedist is a fellow of the academy of sports medicine (or whatever it's called).
My two spine surgeons, my neurosurgeon, my two neurologists, my two physical therapists, and my orthopedic surgeons all said the same thing: loading your spine risks spinal disc damage, nerve damage, joint inflammation and damage, microscopic ligament tears and bone fractures, and a host of other issues. Yeah you can get "gains," but you can also get very bad things, and it's a matter of luck/mistakes, anatomy, skill, genetics, and other factors that can cause those bad things, so as preventers of disease, doctors will always recommend the least risky form of activity that maximizes health, not size or gain.
I will not listen to trainers over doctors. I injured myself with trainers. And then I had another trainer a few months ago offer to train me and he gave me advice that was contrary to my doctor even though, like I said, he himself had suffered two neck surgeries from damaged discs. So I will not risk my body for a certified trainer anymore.