That's why they should be phased, and not too steeply.

If they're phased, e.g. at 30% (for every additional €1 you earn, benefits decrease by €0.30), you have the problem that when you are applicable for several of them (e.g. for children, child care, chronic illness, etc.), the benefit reduction adds up as well, so you're quickly back at an effective marginal tax rate (EMTR) of 90% or even over 100%.

You'd think that it wouldn't be beyond the capability of our society to declare that "the EMTR shall be at most 70% at any point in the income curve", and do the math to make it work, but apparently not.