actually lizzy its EASIER to train a deaf dog than it is to train a hearing dog...
the biggest challeng is recal since they cant here you and most people training deaf dogs for recall use a vibrating collar (or ligths at night). but hand signals dont have to be "official" (i make most of mine up when i was working with my past deaf dog and the deaf foster dog) and dogs learn hand signals MUCH quicker/easier than they do human words.
blind dogs just need ot be trained on verbal commands and need a person whos willing to build confidence through guidence (the dog must trust its owner) and one whos willing to keep their house layout how it is and keep a routein...most blind dogs learn they way around a new place in no time, ok so they bump into a few things in the beginning but there no harder to train than having a dog who can see...
my friend has a blind aussie who runs agility, obedience and frestyle...in truth if you didnt look at her face you would have absolutly no idea she had no vision (shes missing both her eyes)
blind dogs are certainly a little more of a challeng than most deaf dogs, but neither are so disabled that they are hard to train...
i think the lessening number of people willing to take on "disabled" dogs is based on 1: having the "prefect dog" people forget that these dogs can be just as if not even more perfect...King, my dane growing up was my angel, he was 100% deaf and blind in one eye but he was smart as a whip and will alsways be my first true heart dog...
and 2: based on the fact that most think raising these "special needs" dogs is going to be millions times more "challenging" than raising a normal puppy...in truth, its the exact same, you just modify the training method a little.
thus far every deaf dog ive had trained easier than those with perfect hearing, and ive never seen dogs bond as strongly to their people as those with vision impairments.