I don't think yorkies are any more difficult to train than any other breed. All dogs require consistency and a good game plan. I think some people find it harder to potty train because they aren't as consistent with a yorkie as they would be with a great dane! Imagine if a great dane pooped in your house, you would do everything in your power to stop it from happening again.

When a yorkie poops it is annoying, but easy to clean up, so people aren't as committed and consistent as they could be.
That being said, there are dogs that are harder to train than others! Doesn't mean that yorkies are harder to train overall, though.