The skillbook will cost extra coupons/ingots, and seeing how much the tobirama/hashirama skillbooks costs...

also his mystery only interrupt one unit not all, and his second mystery lvl doesn't interrupt at all, i prefer spaming 2 interruptions each turn instead of 1 interruption or full team damage every 2 rounds, one more thing on auto battles the 2 interruptions are much valuable because you can basically nullify any possible mystery on any given turn just with that, on the hokage side, your tags are all random and you might end up never doing his second mystery lvl, between AI wrong targets, immune ninjas and debuff removing/mirror return ones

PS: about the interruption part thing: source: https://youtu.be/bjFSuN1ptBE?t=184