There was nothing easy about the Raptor engine, for one. It is absolutely the best rocket engine in the world by far, and the only methane-based engine that ever reached space.

AFAIK the only "real" problem that SpaceX is now having with Starship is the heat shield vs. rapid reusability. It is an important problem, but it also means that many other complicated problems (such as precise exercise of the belly flop) are fully solved.