There are myriad reasons why projects go over budget and/or schedule or outright fail.  While many factors are involved, the importance of a properly sized and experienced system engineering team with properly tailored processes and tools cannot be understated.

DAVID - This is a topic that seems to come up a lot, and generally when there is professional friction going down at work.  Hardcore design engineers, when schedule-crunched or technically challenged, will find ways to subtlety or not-so-subtlety imply that Systems Engineering is hardly its own discipline, and organizations that treat SEs as such are burning money investing in glorified project managers.  Others in leadership take it a step further and flat-out say, everyone who is an engineer on this project/program is a "systems engineer".