I have heard that fenbendazole works very well on koi symptom free. I have never had the need to use it on my koi, but used it on some of my goldies after I used a lackadasical QT treatment on some new fish that brought in flukes. Everything measured with a gram scale. It worked great on the flukes but killed half of my goldies, one of which was 6 years old. They started going downhill 2 days after treatment, and within 2 weeks if they were not dead I had too euthanize. Lesson learned.
Regarding prazi, I used it a couple of years ago on koi. At the time I had about 10-11 fish in 4500 gallons. One treatment worked great and every fish came through great.....but one. It was a female kohaku that looked like it was burned. The white skin turned very pink, and the beni looked like it was going too disappear. The fish was listless for about a day or two. The other fish were FINE like nothing ever happened. Eventually the kohaku came around and was fine. Its beni even started getting thick again.
I can't explain what happened but just thought I would relate some first hand experience I have had with both products.