There are, formally, two types of royal commissions -- investigative commissions and policy advice commissions. But in reality there are those you can't avoid politically, and those you think will hurt your opponents.
A investigative royal commission, of the kind the government will announce today into trade unions, assumes some major systemic failure in a regulatory system, whether it's police corruption, whether religious groups turned a blind eye to sex abuse, or the failure of financial regulation. In a well-ordered democracy with the rule of law, investigative royal commissions should be few and far between -- normal regulatory systems should function with reasonable effectiveness. And in practice, because of their expense, length and unpredictability, governments are reluctant to instigate royal commissions, particularly if the systemic failure being investigated might be close to home. There's never been a royal commission, for example, into how Australia joined the attack on Iraq on the basis of a falsehood, which cost taxpayers billions of dollars and served, as is now widely agreed, only to make Australians less safe.
But, naturally, none of that applies if you regard royal commissions as political tool with which to pursue your opponents, which is a Coalition speciality. It was the Howard government that went after the CFMEU via the construction industry royal commission, and insisted on a second royal commission into the Australian National Audit Office's Centenary House lease in order to attack Labor.