One serious danger I haven't heard anyone mention mention in regards to the Mars colonization project (though it was considered to some extent in Arthur C Clarke's "Contact" and probably some other science fiction) is the possibility of terrorism against either the fragile spacecraft or colony.

Musk has said there will be no screening of the Mars colonists, and that anyone could go. That means someone who's mentally unstable and/or wants to make a name for himself (ala Herostratus[1] or any number of modern publicity-seeking terrorists and murderers) could go and attempt to harm the spacecraft or colony, both of which would be incredibly vulnerable to such intentional attempts at destruction and are guaranteed to get massive publicity were they to be destroyed or even merely attacked.

This could become even more likely if living on Mars long-term actually becomes viable, and people wind up spending decades on there. Some people will likely go stir-crazy and attempt to harm themselves and/or others.

People who are allowed to go live in Antarctica or out in to space are currently screened very carefully to be compatible with each other and able to psychologically withstand the rigors of life there, and the relative isolation. But there will be no such screening for the Mars colonists, according to Musk, and the isolation and danger on Mars will be even worse than it is in Antarctica.

The isolation and danger will be hugely stressful and difficult to deal with over the decades people will live on Mars. I've read that even in Antarctica, people are rotated out within a year or so because of the psychological difficulties of living there, and no one's been in space for much more than a year.

[1] - https://en.wikipedia.org/wiki/Herostratus