Not that common. Most companies like to think that they don't have any "bad" projects; if they did, they would cancel them. And, they're afraid that employees

Not that common. Most companies like to think that they don't have any "bad" projects; if they did, they would cancel them. And, they're afraid that employees will confuse "hard" projects with "bad" projects, and try to flee the hard projects, which are often business-critical. Google is smart enough to realize that employees can often identify bad (and good) projects before the management does, and lets them vote with their feet.