Endangered and Threatened
Some Heritage breeds are good, solid breeds, which are readily available, and which do not have issues from being badly revitalized. Others are currently still endangered or threatened, some critically so.
Animals that are more endangered are harder to obtain, and it is more difficult to find good ones.
That said, this is where the effort is most needed. And this is where great care is most important.
Attention to breeding quality, good care and good nutrition, and development of quality standards for your farm are more important than they are for less threatened breeds.
If you enter into this arena, do so with full commitment, because your attention to breed preservation may make a difference in the survival of the breed.