I keep a couple hundred birds on our farm. They are great for scratching the manure and eating any excess grain and bugs. I don't have a fly problem. They clean up spilled feed and this helps prevent rodents. I've seen them hunt down and kill mice. They provide eggs for sale which helps pay the feed bill.
The problem with chickens is that they do poop everywhere. If they get poop in the feed or water pans they could possibly spread disease. There are a lot of diseases that can be spread by poultry. Making sure you have healthy, parasite free birds is key.
Spent a whole hour today laying in a pasture, waiting for a sparkling vampire to show up. Alas, I woke up and looked over, only to find a mound of horse crap. Sigh.