I definitely get what you mean by burning out. Even on project that is long, it is easy for me to lose interest. With LabVIEW it always feels like getting 90% complete is pretty easy, quick, and there are plenty of results to see. That last 10% seems to take so long to wrap up, and so much work that I find myself just putting it off and doing other work instead. Of course when timelines are involved, and you need to actually work on it, it is hard to stay motivated. Maybe its hard because I enjoy doing LabVIEW and it doesn't feel like work when its fun, but when it isn't fun it is a drag.
For large multi member projects it gets a little better since you can all share in the accomplishments of everyone, so staying motivated is easier. From a program management stand point I've seen plenty of projects eat up 8 or more developers full time for months (years?) more than originally planned due to poor management, and unclear requirements. Those are pretty bad sometimes too because the moral of the team is poor. Constantly getting beat up, while working hard can make good people want to quit.