We have a lot more experience building planes of various types that can fly. This is well understood science. The difficulty with rockets are the razor thin margins and the extreme conditions it needs to handle.
The science is probably well understood. But getting a plane approved in different markets must be a massive undertaking. I work for a medical device company and even getting a simple device to market takes a huge amount of money, effort and experience. Now scale that up a few hundred times and you have something that only somebody like Boeing, Airbus or a state sponsored company can do. Add to that changing oil prices and other factors you need a lot of money to finish that effort. Virgin is not even flying their Spaceship Two yet and that certainly is a much simpler development.
Don't get me wrong. I would love to see them succeed but it seems really, really, really hard. Much harder than anything any of the current unicorns has achieved.
It seems to me that most of your doubts stem from the financing side of things and not really from the technical ones. That's a valid point.
From a technical POV, I don't think this is anywhere near unicorn territory. In fact, I think they are aiming too low. I'd ideally want a supersonic plane that is quiet enough to fly over land and cheap enough that your average traveler can afford if they want to trade price for speed. Full autonomous capability so that the plane itself can have thousands of flight hours under its belt before a human test pilot gets on-board.
