That has also been my experience with my own ducks. It has been very hard, emotionally and financially. Like Cori, I also read about ducks being hardy before I got them. My ducks aren't aware of this trait they are supposed to have.
I don't know what your ducks' environment is like, but you might consider making sure they are always standing on soft things. My veterinarian told me that even wood shavings are too hard for some ducks. I thought she was wrong, but when I switched to straw all my bumblefoot issues disappeared and never came back. I also cleared out all rocks, pea gravel, boards, cinder blocks, ect. The only thing my ducks had access to was grass or 4-6 inches of straw or hay.
Of course, my ducks still get reproductive issues. But we no longer had foot issues.