Earlier quoted context omitted.
From [4]: > Rolls-Royce's work over the last few years So Rolls sank several years of investigation into it before cutting their losses. From [3]: > Boom aims for production of the engine to begin in 2025 at the Overture factory at Greensboro, North Carolina Mark your calendar ...
It seems developing a new engine a relatively rare, difficult, expensive, and risky endeavor in aviation. Notice none of the aircraft companies make their own, right? It's CFM, PW, and RR. But Boom has a bunch of propulsion engineer openings so it looks like they're really going for it.
Sure there are counterexamples, but they have good reasons to think that this is more than double the difficulty of developing one of these parts. And that the engine will take longer.