It is very counterproductive when you are, for instance and staying close to home, managing a group of engineers whose job it is to know exactly what is going on in a project. Realism is appreciated, motivational talk is laughed at, as most are inherently motivated to do as good a job as possible anyway.

Most engineers I know only stop giving a shit when they have become convinced on technical grounds that something will never work, yet are forced to continue. No amount of motivational talk will cheer them up, they will just get more annoyed, and you can forget about the coveted happy smiles all around.

Again, I'm speaking from personal experience only, I haven't researched this.

As an anecdote, I remember a project being named "Marvin" by its project manager and engineers. Management didn't pick up this was a reference to the depressed robot from The Hitchhiker's Guide to the Galaxy. It's the sort of too-subtle protest people are forced to use in an environment that thinks positive thinking conquers all, and blunt realism is frowned upon for not being cheerful enough and can even get you fired.