My daughter is allergic to cinnamon. We gave up breakfast buffets years ago because there's too much contamination , both by guests switching tongs and in the kitchen since it's so fine and goes airborne when baking. At WDW buffets, they always just bring her a special sundae because they can't guarantee that any of the desserts are safe.
I usually don't even mention my fish allergy because of course I'm not going to order anything with fish in it, and I do my best to avoid restaurants that even serve fish when possible, but I think I'll note it on the ship since I keep reading about all the seafood dishes they serve. I don't want my chicken breast cooked anywhere near somebody else's salmon!