Generally, it's easy for someone to mess up any project, Unity or not. What matters most is that people learn how their project works and what could mess it up.
Unity, on the plus side, is very easy to learn. I've worked on three Unity-based projects, all with small teams of 5-10 people, and I've never seen anyone mess the project up... much. I mean, there were some misunderstandings, wrong files committed to source control, etc; but nothing that couldn't be fixed in half an hour tops.
Unity is pretty easy to use, and one of its strengths is that it allows your audio/graphics guys work in the same editor as everyone, and easily test their work in-game. Being able to immediately see one's work ingame is a big boost to productivity. Also, Unity is very easily extended. You'd probably find yourself coding dozens of little tools to help both programmers and artists with their work, and this will probably add up to big savings.
Regarding source control, don't even think about Asset Server, it sucks (AND costs money too). Since 3.5 Unity supports external source control even in free version; and also text-based asset format that is more-or-less merge-friendly. I've successfully used SVN with Unity projects, and I believe Hg or Git would work like a charm too.