I think Yorkies are harder to housetrain because of three things:
1. It takes longer and people give up. Most need consistent training until they are 10months-18 months old before they are fully trained.
2. They are small and they have to go out a lot, frustrating owners and giving them an opportunity to have accidents.
3. People don't treat them like dogs - if you had a lab would you tolerate peeing on the floor or pooping in the closet? Not a chance. Crate training and outdoor training are perfectly acceptable. Would you let a lab use pee-pads? Ha!
My two are housetrained and they ring a bell to go outside and go out on leashes even when its 0 degrees in the middle of a Chicago Deep Freeze. Loki's 4 and he's survived so far! House training is no fun and it's like having a 2nd full time job. If the bell rings when your favorite tv show is on, you have to jump up and take them out. You have to watch them every second they are not in their crate, carry treats in your pocket and shove one in their mouth the second they pee outside (so they know why they are getting the treat!) and it helps to invest in a good steam cleaner for the first 10 months or so
If you are consistent you'll be fine. First they start to only have accidents every couple weeks and then when you think they will never get it, they do!