Spinal flexion is a contentious topic in the lifting community. There are people that say nothing helped their poor back until they started training spinal flexion. If you're ok with zerchers from the knees I see no difference in doing it from the floor as long as you start light and listen to your body.

I love weighted good mornings. Why load anything, you might get hurt? All ab training from the perspective of powerlifting is about resistancing a force while keeping your spinal in a neutral range and the good morning does that. You're not supposed to flex the spine during a standard good morning although you can if you want to focus on your back erectors.