Just because you can cut immobile chunk of rocks doesn't mean you can easily cut a mountain, especially when that mountain can actually attack you from everywhere. And considering that while assimilating stone, it's difficult to do significant damage to the original Pica, the sheer scale of Pica's attacks will completely overwhelm Inazuma in the long run and will end up getting burried alive in the stone. Hell, Pica probably doesn't even need his stone assimilation to defeat Inazuma when in his FBH, he could crush him