There's also some concern for what life will actually be like for any poor sods actually stuck on Mars. Between the harsh conditions and what we know about the aspiring Emperor Musk of Mars, it's likely to make the Sea Org look like Butlin's.

Relatedly there are questions about why the whole project seems to assume that it's much more important to preserve the human species, like a celebrated breed of dog, than to protect the lifespans (ideally, healthy lifespans) and welfare of individual human beings, questions which unfortunately can also probably be answered with reference to Musk personally: https://youtu.be/-VfYjPzj1Xw?t=18 .

There's also the wrinkle that if it's viable to send settlers and adequate resupply to Mars, then it's probably much easier and more straightforward to send a few nuclear warheads on their own one-way trip, so the whole premise that a Martian colony wouldn't be struck in a full-scale nuclear war on Earth seems shaky. Now a Martian colony dug in deep under the surface might be relatively safe, but that brings up again the question of whether it wouldn't just be better to build a far-flung network of underground settlements on Earth and/or have submarines loitering in the Antarctic Ocean in shifts.