Supersonic is “easy” in the sense that rocket design is “easy.” Orbital rockets were still out of reach of non-government-funded efforts until SpaceX, and supersonic flight is still the sole domain of government contractors now. Boom is changing that.
Easy of course in the sense that that many aerospace engineers and aircraft have done it all over the world for many years. And most "government contractors" in the capitalist world are civilian private companies, many of which build both military and civilian aircraft and started small.
Which means, for example, that even this small private company knew pretty well what to look for in wind tunnel tests and other materials work. Their first transonic and supersonic flight was stable, did not destroy the aircraft, did not kill the engines, etc. Even, presumably, broke through the sound barrier the first time they tried - and was fully expected to.
