ULA's plan is to recover only the engines, so the other bit to go wrong is the explosives to separate the engines from the fuel tank. ULA are a bunch of rocket scientists so they have the technical skills for mid-air retrieval. They would need an astronomical budget. As they have a proven record of getting billions from the US government, I would not scribble 'impossible' on their plan. I would go with 'ambitious with a clear smell of desperate', 'expensive' and 'will get delayed at least five years'.