[Autumn]

Fight As One's range can easily be upped a little bit I would have to say, but the problem with multiple bonded youkai is there's little reason to summon multiple bonded youkai, because of how Shared Pain works if you and all 3 of your youkai get caught by a single AoE, you share 50% of the damage 4 times, leading it to be 200% of the original value.

Fight As One's effectiveness is more useful to the summoner than it is the youkai themselves, so another reason people go 1 bonded youkai is because +30 to hit/crit/evade/crit evade is objectively better than 15/15/15/15

You would improve Fight As One's effectiveness with multiple bonded youkai by increasing the range minorly (like 5 range) and then making the effect stack per bonded youkai in range, causing it to be better if not way more risky/resource consuming that a single bonded youkai.