The maternally derived antibodies will only last in a foal for 3-6 months, so if you can get the foal through the first 6 months of their life without any issues then they should be just as healthy as any other horse. In the first 6 months the only real way to replace the MDAs is by plasma administration.
There is a theory that pemphigus foliaceous is due to not recieving sufficient colostrum from the mare when born... Not sure how well evidenced it is or how your could prove it
