As a breeder and exhibitor, I'll attempt to answer your question from that standpoint.
First there is no such thing as a teacup yorkie. It is an advertising ploy set out by unreputable breeders to obtain more money from the unsuspecting/uneducated public. Reputable breeders will not sell these wee ones, but place them in qualified homes that are very aware on the care they require. No, not all are unhealthy. Unhealthy can exist in anysize yorkie, very dependent on the breeder's breeding practices. However, reputable breeders do not breed for tinies, but they can happen, just as an oversized yorkie can happen. In addition no reputable breeder will ship these wee ones.
Even a reputable breeder who tests for all the genetic problems that may exist.....you cannot know if a a pup only has one kidney, or other internal issues that cannot be determined unless surgery is performed. That is why reputable breeders do not sell them....they don't want to sell heartache. Placing them in qualified homes that understand that they may incurr vet bills.