First thing first, Masamune is strong so let's make that clear. Being
the Youkai king though, you kinda expect more from him. His Eye-drop skill
that he uses against you won't be there after he joins. Without any skills to
provide variety, the only thing Masamune can do in army battles is hack away
at your foes using his superior stats. That is not ineffective by any means,
but quite a few of your warriors can do just that and offer more on the side.
I tend to be strategic when I can so I don't appreciate Masamune's simplicity.
He's an absolute beast in solo battles though. High level, high hp and a monstrous 6 actions