What are some concerns of mother's using breast milk instead of formula ?

Only a few things: At first, i couldn't think of any, the designer really knew what babies need & it's worked for hundreds of thousands of generations. The only exceptions i could think of are if mom has HIV or untreated tuberculosis.Moms on bizarre diets can also be a problem but that is also rare.

Another Concern: There are a number of drugs that can pass into breast milk. Speak with your pediatrician about any drugs you are taking and they can tell you if they pass into breast milk and might be a concern as to the effect on the baby receiving breast milk.