Musk has been very clear that the ITS is not a complete colonization package, it's just a transportation system.
That said, we have decades of experience with habitable spacecraft. Mir was inhabited for over 12 years, and the ISS has been inhabited continuously for nearly 16 years. That part of the technology we have a pretty good grasp on.
We really don't! Mir and the ISS house only a few people, and are constantly being resupplied.
We have no experience with building, or maintaining self-sufficient - or even somewhat-kinda-sorta close-to-self-sufficient biospheres. Without them, Mars is a pipe dream - it would be far too difficult to maintain any kind of colony.
Unlike building rockets, the thing with ecology is that we don't even know what the hard parts are going to be.
That said, we have decades of experience with habitable spacecraft. Mir was inhabited for over 12 years, and the ISS has been inhabited continuously for nearly 16 years. That part of the technology we have a pretty good grasp on.