I'm a consultant usually brought in by companies doing project management on big projects, setting up operations etc. The last few years I have been dealing mainly with digital and creative agencies.
This sector historically has a low level of expertise within software development. The lifespan of agency projects, once released, are often measured in months rather than years, since most projects are in some way marketing related. As such they are not accustomed to the complexity or scope of "real" software development.
My problem is as follows: When presented with a new project, which I can rather quickly dismiss as unfeasible (we're talking extremely unrealistic projects), I have trouble communicating why it cannot be done. I argue the technical complexity and the requirements versus the time and price scope. I am however not able to make my points persuasive.
Essentially my recommendations get ignored, and the client looks around until they've found a vendor or individual who says "yes it can be done". Both times, the projects crashed and burned. Reason being they were so amazingly unrealistic, that even inexperienced technical project managers would dismiss them right away.
What kind of approach or what kind of arguments would you use when presented with something unrealistic, yet highly desired by major stakeholders? Maybe any literature to read on the subject?