I trained Jackson when I was not working and on my college break. Maybe this is why his training stuck and we never had issues because I had a solid month off to be with him and train him, and it was just he and I. Which I understand is an unusual situation for most people. We had a routine where I'd take him outside, reward him when he went. If he didn't go potty while outside, I'd put him back in the crate. Try again in 5-10 mins. Don't let the dog run around freely or not under your very watchful eye at ALL at first. That's when accidents happen (and it happened a few times in the beginning with me where I'd look at my phone for a minute and then see him go over and squat LOL) but some accidents are to be expected for sure.
But is he 100% potty trained, not even an accident when he is sick, he will go bark at the door when it's urgent. And like I said above, I would never ever scold him for it, or care if he couldn't hold it while he's not feeling well, but he still refuses to go inside. So it is totally possible to have a 100% trained Yorkie. And yes he's a bigger one too (18lbs) but my friends 4lb and 6lb Yorkies are 100% house trained as well. She told me the only time they have accident is if they've had upset tummy or something, but I think that's common with a lot of dogs, not specific to Yorkies.
I honestly think it's more common in smaller breeds to have housebreaking troubles simply because they can hide it so much easier and most people give too much freedom too quick. A large breed peeing or pooping in the living room is hard to miss. A little Yorkie's pee spot will dry up quickly and half the time you won't even know it was there lol. In the beginning I can't stress enough to literally not let them out of your sight. If they are not in the crate, they are attached to you via leash or holding them. They only get down to play after they have done their business outside (and you witnessed it). And even then, after a 30 minute play session as a really young pup, they most likely will have to go again after all that action/movement.
I would say expect them to take a bit longer to train than a larger breed but they are not at all impossible with dedication
Jackson often sleeps in on the weekends with me until 10am or later, he has not been in a crate since he was 5 months old and has free roam of the house. If he has to go, he lets me know.
I try to make sure he gets out every 6 hours at minimum during the day. Overnight he is good for 8-10 hours.