Shelled sunflower seeds aren't going to cause any trouble. Chickens love them in any form (whether raw or cooked, unsalted, salted, or flavored). I doubt they can manage sunflower seeds in the shell, though.
In fact, chickens can have just about anything! It's hard to go wrong if you feed your chickens something in addition to a high-quality chicken feed. If you try to feed them something questionable instead of a high-quality chicken feed, or mix something questionable in with a high-quality chicken feed (perhaps spoiling it) -- that's when things go wrong.