Lagoon and Atlantis bonus is nice and useful even if just have a pair of two. The Sakura family almost never profits from it’s bonus as it only has a effect at certain breaking points.
The bonus is 2%-4%-7%-12% for 2-3-4-5 sakura heroes respectively.
Kageburade with very fast mana needs a 9% bonus to profit from this. Average Mana Heroes need a 12% bonus (if my calculations are correct). If you have no troops with mana bonus you’ll need a full team of sakura heroes for them to gain any profit from this. Sakura heroes with fast mana don’t profit from it at all (or only if you have the mana troops).
If you have played for a while the 3* and 4* heroes are only used in challenge events, but since you can’t pair them with the mana troops Namahage and Ameonna don’t get the bonus.
In certain situation combining the mana troops with the family bonus can help to reach an additional breaking point (e.g. reducing mana costs for average mana heroes from 10 tiles to 8), but you need at least 4 sakura heroes (ideally legendary ones) for this and leveled mana troops.
Without a full team you’ll only see the effect if it pushes the troop bonus above the first breaking point (e.g. with lvl 5 mana troops + 2 sakura heroes Kageburado would need fewer tiles for full mana. With level 11 troops the family bonus becomes redundant for him, but could help average mana heroes).
So unless you have a full team of them, you’ll only get the family bonus in very specific situations. In comparison to Lagoon and Atlantis family I find it rather weak and unreliable.
I think it would be better if they would get additional health or a crit chance instead of the mana bonus, as those are more reliable and useful to all players wiht sakura heroes.