It can lead to some unintended consequences if it is bread or things of that nature like duckling malnutrition and loss of natural behaviors.
Always check city ordinances that restrict or prohibit feeding ducks or waterfowl, but grains, oats, rice, lettuce or greens chopped up, or especially birdseed any type or mix are all generally okay.
Uncooked oatmeal is an excellent source of nutrition for birds, and it also helps you get rid of the oatmeal you are not going to eat.
It's fine. Be careful as, depending on where you live, you can attract unwelcome guests (rats, raccoons, bears and such)
It's fine. Just don't give them things like uncooked rice - it expands inside their tummies (that's why they don't want people to throw rice at weddings anymore unless they clean it up - the birds eat it and it can kill them).
I know they really appreciate it in the winter.
I would think as long as it isn't raw dough it is ok.
I doubt it's bad for them. I remember when I was a kid I'd go out and feed the ducks with stale bread. The birds would love it this time of year.