Personally got 2x hashirama during 2 mobilization event without spending any coupons on it (so 140boxes in total spread across 2 events, 1 hashi/70boxes). At least 2 other people got him as well the same, free way. Got gnw kiba from lucky board box as well recently (although everytime theres lucky board I use about 2k coupons on it, for last 6-7 months) and he's my first ninja from lucky board.

That being said, there was a video with 1500boxes and no ninja, so you see how vastly it varries. It's a lottery, gamble- except you cant tell your chances. If you are feeling lucky, go for it. If you dont feel like it, don't. It's entirely random.