I feed my dog human food, but ONLY under certain conditions:
1. Never a large amount. Usually no more than I can fit on the tip of my finger.
2. Only "basic" foods that I know the ingredients for. e.g. bread, fruit, plain cheese, plain pasta
I would not feed her, say, pasta sauce, or crackers with seasoning on them, because it's too difficult to know all the ingredients there.
3. I always Google it before I give it to her for the first time. Everyone knows that dogs are allergic to chocolate, but they're also allergic to other random stuff (grapes, onions, avocado for some iirc). If there's any doubt, err on the side of caution. No need to mix up your dogs diet with exotic human foods; they're happy enough to be treated with a bit of peanut butter.
So yeah, tl;dr: I wouldn't feed my dog pizza. Too many ingredients and too many unknowns.