We have two different breeds so-
Charolais: proven cow families, consistent bull (not bull of the month either), a guarantee on conception and now PAF on donor and/or Sire. I have also been burned before on buying through an auction with no guarantee (0/5 resulted in calves on one flush I bought).
Murray Greys: proven cow families, outcross genetics, cost, free of myostatin, proven temperament, frame size 4-6, and all paperwork is complete (its pretty extensive for MGs). Very few if any MG embryos will come with a guarantee so I am being more careful about who I buy them from and try to have them shipped direct from the collection facility.