The way we see it, unions are best used when there is a power discrepancy between employer and employee. In the end, your value as an employee is how much it costs to replace you. Simple as that.
So if you are educated in a specialized field, you have bargaining power (and you don't need a union). If you are working in a more generalist area, you have none whatsoever - it is an illusion (exception being if the demographics means 100% employment).
What the union does is of course to band generalists together so they have strike power. That sucks for the company. But what the Danish unions tend to do is demand eduction, lifting the generalist work into a more specialized position. You can always get good at a job.
This means that the company may have to pay a more expensive salary, but get a much improved worker and/or product. Paying 50% extra for twice the productivity is a good investments, even if you have to pay for some of the education of the former generalist.
So rather than just demand more pay, the unions demand more education and higher standards. They also demand better pay, of course - but in the end they are interested in many members working rather than few members getting ridiculous payments.
Mature unions are good for the economy. Corrupt or immature unions can be a terrible ball-and-chain for a society.