I Don't think Yorkies are the hardest to potty train. IMO i think it depends on how much time you are willing to put forward into potty training your dog, whether it be a Yorkie or not.
I got my first Yorkie/Pomeranian at 8 Weeks old and he was doing great with his potty training the first week of training.
My 2nd Yorkie I got it took her only 3 weeks to potty train, and my third (maltese) it took her a while only because it was ME who couldn't stick to our potty training schedule.
They are all potty trained right now with few boo-boo's but it's expected (well from me at least, because i make mistakes too! lol
)
this is just my experience.