On this years RAGBRAI there were a lot of people on mountain & hybrid bikes. On the downhills some of them would lean way forward over the handle bars to get more aero. These people were so far forward they could easily rech down & touch their front tire with the palm of their hand. This looks dangerous. For one their center of gravity is pushed forward as is their weight. Though there were no accidents I saw as a result of doing this I could just imagine the result if a person on an mountain or hybrid getting aero by leaning forward hit a bump & went flying over the handlebars superman style.
If you want to get more aero that is fine, no problem with that, put aero bars on your mountain or hybrid. This allows a more aero position but keeps you from going to far foward liike I saw some of these people were doing. Ride a road bike & use either the drops or an aero bar. Or better yet ride a recumbent. But you should not put your weight or center of gravity that far forward to get a more aero position.