Dogs will pee and poop outside and they have dander which are the two big complaints people have (for the most part).
I have witnessed countless adults and children not wash their hands when leaving the restroom, cough, sneeze, pick their nose, etc and then put their hands on the handrails, coasters, buggies, FP+ machines, turn styles, door handles, etc that you WILL touch at WDW.
Why are people acting as if dogs are so disgusting? Humans can and mostly are far more dirty than your average dog. I was staying at Boardwalk a few years ago and I walked into the restroom next to the lobby and their was a man cleaning a pair of kids pants that had been pooped in. The sink was FILLED with dirty brown overflowing water and getting all over the counter. It was one of the most disgusting things ive seen an adult do at WDW. I think people acting as if dogs will turn WDW into a toxic dump are overreacting. You are exposed to more germs and contagions from humans than you will be by a dog.