Per the title, the SpaceX BFR has the following apparent problems :
a. It’s an enormous vehicle and would have to be hand built since there won’t ever be very many of these. Thus no economies of scale in building them.
b. Every time a BFR gets destroyed, it’s a huge loss. Once they are crewed, it could get a hundred people killed all at once.
c. The sheer size causes costs of it’s own. A bigger factory would be needed, a big assembly building, just big everything.
So the obvious alternative approach would be for SpaceX to standardize on a single type of rocket. Perhaps something a little larger than the current Falcon 9, with the upper stage using methane. This single rocket type would be used to make many shuttle runs, gradually assembling an interplanetary excursion vehicle in space.
Presumably the interplanetary excursion vehicle would be made by first launching a series of expanding structural trusses and a long robotic arm. The trusses would be put into close proximity by the arm, and servo driven bolts would join the pieces together. Fast forward for a few years of launches, and eventually a complete interplanetary vessel is ready.
Hmm. I think I may have answered my own question. What happens if something goes wrong in the process of putting together these lego-like modules? A bolt freezes, a connector gets jammed. SpaceX would have to then launch astronauts, or very capable robots able to do the EVAs to fix the issues, which have not been developed.
While on Earth, the entire BFR can be manufactured and assembled and tested together as a complete system.