Ok some possible down sides.
If you get a softer coated Yorkie you'll need to do lots of grooming. They require more time in house training that a larger breed. They can be barkers (not all but some are). They are bed hogs and blanket stealers as well.
Health problems to look out for are Liver shunt, MVD, Allergies, LP, Hydro, Thyroid problems. Of course this is why you need to speak to your breeder in length prior to buying a pup.