that technique has its down sides, as do all.

it requires the person to hold the other while his soul is removed, and like the 3rd it requires a lot of stam/chakra to actually steal the soul. it would be useless against a high speed ninja untill hes court, and even so its useless if the person is strong enough to break free of the hold.

id have to say the black flame too, since not even the perverted hermit knows what it is, and hes very knowledgable, its something hes never seen before and it broke free from his technique which he said no ones ever escaped from.