It changes every time your salary changes and when you take unpaid leave, even if only for a few days. For a dual income household this is already a bother because of the details you have to enter. For households with lower incomes it also doesn't stop with child care subsidies, but also a health care, rent, and an additional child specific subsidy to manage. Now consider that many people in lower income brackets do not have a fixed salary but one dependant on hours worked (think retail staff etc.), and you can almost guarantee that you will receive too much at some point, which will be reclaimed.

Compared with the ten minutes filing taxes takes here (with everything prefilled and rarely a need to change anything) this system is a high-risk travesty that wastes a nation's time.