There are of course exceptions. Mothers with advanced HIV or Hepatitis B, will be asked not to nurse their baby for fear of transmission. This also applies if the mother has a herpes outbreak on the breast or nipple. This too can be dangerous to an infant. Mothers who are active drug users or abuse alcohol should not breast feed.
Babies can easily become bored by tastes.
With baby formula, the taste will remain the same. The baby no longer desires the formula as much as it had, and this may cause problems. Breast milk on the other hand, changes in taste day by day. It alters itself according to what the mother has digested. No formula on the market can compete with mother's milk, but do not feel inadequate if you cannot breast feed.