Chickens suck because they poop on everything, and it dries into a glue-like substance caked onto things. The straw ends up caked in poop, the walls get caked in poop, the floor gets caked in poop, the chickens poop on each other. Getting it off requires a paint scraper, and getting way closer to it than you want. It's also liquid-y. It's a lot like bird poop on your car, but bigger because the bird is bigger.
The horses were less bad. Their poop was fairly "clean" as far as things go. They stayed pretty structurally intact (it's basically a ball of half digested fiber, kind of like a hairball) so it wasn't a big deal to get them with a pitchfork, and they were almost exclusively on the ground. It's not a job I wanted to do, but it wasn't awful. The heat in the non air conditioned barn was honestly worse than the work.