Last three times I had KFC meals I had not-so-pleasant experiences in the toilet soon afterwards. Which is a shame since I loved eating at KFC once or twice a month. Had this experience with McDonalds a few times as well.
Decided to try out recently opened (near my home) Burger King and their burgers don't upset my stomach so far. And the quality of the ingredients in itself feel much better. But I imagine they differ at fast food chains here in the Siberian city I live from what's used in other regions.