Radiation is the biggest uncertainty IMO. The issue is in transit though far more than on the ground. Shielding on the ground in a hab is relatively easy. While out exploring is tougher. I haven't looked at the mechanism of damage in detail, but have heard people who have relevant expertise say it's different. What I have looked at is shielding and people frequently miss that it is simply the mass of Earth's atmosphere that does most of the shielding and that is not equivalent to all that much solid mass. The magnetosphere is effective stopping particles from the sun, but not cosmic rays.

Mars Direct is not a colonization plan. It is an exploration plan. This is why I say Musk and Zubrin are complimentary not adversaries. Musk is working to reduce launch costs to the point where the private missions to Mars needed for colonization are affordable (or at least starting that process). Zubrin is trying to get the government funded scientific exploration going that will as a side benefit help develop the technology to live on Mars through the use of local resources.

I know there are many points of Zubrin's plan that are debatable, but I think too much is hand waived away as if those doing that have much better insite when in fact they are all just making their best educated guess. I don't like dismissing the tether concept without even trying it and I don't like leaving your return vehicle in orbit for 2 years. Maybe they feel the dangers of space are less than the dangers of dust on Mars, but I like people being able to keep an eye on and repair if needed their return vehicle. I don't like that we have never even tried to measure the long term impact of partial gravity.

Regards,
Mark