>For example, if you have a real estate bubble you have a strong incentive to show population growth.

That's one source of bias that is present at a specific time. Mostly you would have competing incentives. There is usually more than one agency that runs does the counting. Vital records registration, voter rolls and tax payers lists, for example are separate agencies in some countries. Not every tax payer is a voter and not everyone who was born still lives in the country. The sources are sometimes cross-referenced too. Then there is usually a place that needs to do macroeconomic forecasting and needs to have some numbers to do it's job.