No, killifish are not hard to keep. While their specific requirements will vary depending on the species, killifish are generally easy to keep.
Killifish need a clean tank with a tight lid (they are jumpers), a water PH level of between 6 and 7, rocks, floating plants, a balanced diet, a water temperature of between 72 and 75 degrees Fahrenheit (22 and 23 degrees Celsius), and a water hardness of 122–162 parts per million (a water filter can help you maintain this).
In addition, killifish like lowered lighting, and prefer live foods like insect larvae, crickets, worms, and crustaceans. Also, female killifish are more passive than their male counterparts and can be kept together if they have enough space. One pair of killifish needs a 20+ gallon tank.