No, killifish are not hard to keep. While the care level for killifish can be easy or difficult depending on the species, killifish are generally easy to keep.
Knowing the specific requirements for your species of killifish is important. Some killifish are picky eaters, while other species will eat many kinds of food. Most killifish are peaceful, but some species are aggressive. Be sure to research the species of killifish you want to put in your tank.
With that said, most killifish are easy to keep because they are hardy. They need stable tank conditions and a varied diet, but most species don’t have difficult care requirements.