Mustard gas and canola have nothing to do with each other. Canola is part of the mustard family (as are cabbage, turnips, radishes, etc)
Mustard gas was called that because it smelled like mustard.
There is a lot of wild rumors that go along with canola, and most of them are a load of bull. It doesn't cause scrapie, it won't cause respiratory distress (unless, you know, you're allergic to it or something) it does not cause an increase in lung cancer compared to other oils, unless you're the poor sucker spraying the round-up.
Ok, on with the down side of sunflower oils, and other stuff. First, i recommend everyone read "Fats that Heal, Fats that Kill" by Udo Erasamus.
Basic run down, non technical: sunflower and safflower oils can help lower "bad" cholesterol, but may also supress "good" cholesterol. They can also go rancid faster than canola and olive oils. The new high-oleic acid sunflower oils have a similar shelf life to canola and olive, but I can't find any reliable information saying if it is more or less dangerous.
The bottom line in all of this is that oils should be used in moderation and should not be heated to a high level.