Another impediment to the UDF concept which I don't think ever got addressed properly - but would be a major certification hurdle for any aircraft mounting one - is whether one considers the 'propfan' to be a 'fan' or a 'prop'.
If the former, how on earth you're going to meet blade containment rules is beyond me. Yet with many more blades than a prop, it's hard to see how you'd claim that the risk of blade detachment and impact with the fuselage or critical systems was low enough.
Underwing would just make this worse.