My mom and my sister hate onions -- they call it "being allergic", but they don't break out or anything, just that one of the chemicals in a non-caramelized onion turns their stomach.
They can both do fine with onion powder, and fully caramelized onions, though, so whatever the chemical is that they take issue with breaks down with enough heat exposure.
Glad I didn't inherit the issue, since there are so many recipes that are just easier to do with onion than without.