I have read and would tend to believe that when chickens are healthy and strong they can handle a small presence of worms in their organism, but that any weakness or imbalance makes them an issue.Yes that is my worry too.
I have used all kind of natural preventives and they didn't stop Vanille and Caramel from getting really sick (garlic, oregano, apple vinegar, verm-X, I tried pumpkin seeds but almost none will eat them.)
I'm not planning on deworming my whole flock every year especially as dewormers are very hard to come by here without being prescribed by a vet. I used a flubendazole medication meant for human babies. However I don't think using the same molecule everytime will prove effective, so I'm planning to get some goat or horse safeguard through my neighbors and use them individually when the need arise.