I do not have any specific experience with kefir itself but I have a few thoughts:
1) It is still a dairy milk product and therefore has no place in human consumption.
2) It is a product of fermentation...
And, I noticed once, that if people are on drugs that can suppress the immune system should "avoid kefir." Presumably this is an indication that kefir, like other dairy products, causes the body to react as though it is being invaded...