this event stands apart from others as it seems to have 2 levels of RNG.

one is drawing a random reward from the board. you may not get what you wanted, but you do at least get something and that something has value. that mechanic is pretty consistent with other events in this game.

the other level of RNG is refreshing the board itself. there are no rewards for participating in that RNG action and you can easily spend 1k coupons or more without ever seeing shisui, never mind seeing him on a good board. unfortunately, unless you are very lucky, you have to get through this level of RNG in order to participate in the other one, and that makes it a much less rewarding game mechanic overall.

well, i guess the question that comes to my mind is, what's the refresh drop rate for good boards that include shisui? because the drop rate for shisui once you have such a board doesn't matter if you can't get such a board in a reasonable amount of attempts.