Temperature. Granted, liquids more quickly convey energy to solids than gas, but a low simmering pot of sauce should be below pot roast temps. Which is fine for a tiny seared meatball, but for a half kilo beef sphere? That meatball is like 5 cm thick. It would take a while to heat up the center unless you’re upping the temp, but then you’re interfering with the sauce. In short, it’s a major recipe change.