Nutrition and a sufficiently large social, group on the other hand, may be a problem
Mars only gets 43% as much sunlight as Earth (https://en.wikipedia.org/wiki/Mars). On top of that, Mars's lack of a magnetic field like Earth's makes cosmic radiation there worse to the point that colonies will likely be underground, or behind heavy shielding aboveground: https://phys.org/news/2016-11-bad-mars.html
One possible problem not discussed would be the giant dust storms that can cover large tracts of the planet for weeks. We don't have this kind of weather on our cradle planet.
Now the real question is whether colonists can/will live on the surface? But then a secondary addition, due to modern technology we now have much better lighting that can approximate natural lighting, even underground.
Which is why Mars is a tougher project than it seems. The ferry part is relatively easy compared to the incredibly difficult challenge of building a self-sustaining stable ecosystem that can survive on a manageably small umbilical to Earth, and the even harder challenge of building a political system that doesn't explode into factional war, regular suicides, and/or mad dictatorship.
Musk is already a little eccentric and doesn't seem like someone with the world's highest EQ. I suspect he's underestimating how hard it's going to be to make a colony work.