But note that for Denmark, it's only correct by default in simple employment cases without notable investment, retirement, transport/house/mortgage related deductibles, etc.

And it's not at all simple once you have to actually check or change details. It's so complicated that the online UI will only show you a fraction of the available fields by default, and they often reset year-by-year, or get weird values. And then there's the whole real estate taxation scandal...

And more importantly than fixing the statement, you have to actually understand the whole taxation situation to make financial decisions throughout the year. How a mortgage partially interacts with capital gains taxation, dividend and income interact with progressive taxation, how your different investments are spread across a number of different taxation types and intervals, how your spouse's tax situation affects yours, etc. etc.

If you don't pay attention to that, you'll end up missing out on the deductibles the politicians promised you but intentionally handicapped to death by making it yet another obscure tax exception stacking on top of the rest. >:(

I'd gladly pay a bit more tax if I could just get it to be ONE flippin' rate.