I think there are two people problems that aren't often talked about.

One is that stuffing people into a comparative sardine can for a few years has all sorts of negative psychological implications that risk the mission and are also extremely hard to predict and or quantify. The submarine force has some guidelines on this. I don't recall the numbers exactly. But the gist seemed to be that even people who are cut out for this sort of thing and are highly trained and motivated have an upper limit on their ability to tolerate that sort of isolation.

The other one is the PR issue. A guy like Elon can probably lose a crew and soldier on towards the end goal, maybe without too much of a fuss. But if NASA loses a crew, there's going to be all sorts of hearings and people demanding that NASA get canceled and so NASA is going to have a far lower tolerance for the types of unquantifiable risks that come with the first people problem mentioned above.