Homemade kefir is just like watery yogurt with a tang to it. It seems very stable and tough, haven't had any wild yeast contamination problems or anything. As long as you keep it's jar clean and feed them fresh milk they are happy. The only problem I might have with kefir is the small amount of lactose that doesn't get fermented by the kefir grains, seems like when I drink kefir I can feel the lactose (whatever's left) getting fermented inside me. So maybe it's time to try making sauerkraut again.
Don't be afraid to make kefir or saurkraut, just follow good instructions and keep things clean. It's a million times better than the crap they sell in stores.