Lol you're too harsh. I was an aerospace engineer at an aviation startup you've probably heard of. We built a jet and received a type certificate, and sold ships. The company still went bankrupt. Boom is trying something harder, but it's not Theranos, or cold fusion. It's still just an engineering problem. They have aerospace engineers and facilities. It doesn't look more like a scam than SpaceX did in its early days when they were constantly blowing up rockets. It's just they're trying something very hard stretching what any manufacturing startup could do.
3. They're a startup in a manufacturing-oriented, highly regulated industry and dramatically underestimated capital needed.
Engineering and economics problem--although properly speaking the former should imply the latter. Given that engineering isn't "We can build something that costs $X to build and operate but there's only a market for $X/5." That's still an engineering failure even if it works outside of cost constraints.
3. They're a startup in a manufacturing-oriented, highly regulated industry and dramatically underestimated capital needed.