Big thing is nobody has tested transferring cryogenic fuel in microgravity, and the testing on the monopropellants have just been small scale. In order for SpaceX’s starship to work it will have to prove this can work to some extent.

Also, about half the propellant used on a mission is getting to low Earth orbit. Here is a map that shows how much delta V a mission needs to get to places in the solar system. https://imgur.io/SqdzxzF?r