I’ve successfully cast 5.5m of #3 nylon on it (the only length I have so couldn’t test longer). #2-#4 FC in any length casts really well. It’s definitely a more tip flex rod, which I think is a good thing for a rod that is probably best suited for honryu tenkara.
Long lines, bigger flies, big manipulations, and bigger water is what I prefer using the Karasu 400 for. It’s still capable of delicate presentations, tho.