Pros: He can spam his mystery, he has an interrupt chase, he has a dodge, and his standard attack ain't bad

Cons: His chase is not the easiest to achieve and you cannot place him into a random team expecting great results.

Unless you use water main with water enhancement, in that case the standard attack damage is pretty relevant and the only reason why he just stacks 5% nin vs 20% atk is because he scales high on ninjutsu while he scales really low on atk so that after 4 stacks he still has not reached the nin level with atk level. The problem is that many people (and i suppose you are one of them judging from your reply) take for granted that a water ninja is meant to work better with lightning main or earth main than with water main. Guess what? It is not so.

@OP: the only contra is that you need to spend 25k for him and is worth it only if you own two additional ninjas that synergies well with him and they are all expensive (examples with water main: kurama/kisame samehada, edo hashi/ 5 star kabuto snake cloak or hokage minato or kurama). That said the event is good so if you want him do it now.