Do hero IVs matter when a unit hits +10 merge? From the hero stat pages there are banes, neutral and boons; will it all be boon stats at +10 merge?
No, the stats will not all be boons.
If a unit has a boon in attack and a bane in resistance, and you have another copy of that unit that's a boon in defense and a bane in speed, and you get them both to +10, then you will see that the stats will differ. Getting a unit to +10 just gives you a boost in two stats per merge.
When you get a unit to +10 merge, it gives that unit +4 in every stat. (+1 to two stats for each merge, until you've got +20 extra points total distributed evenly across five stats).
So a unit that has -atk and +spd, they will still have less attack and more speed than other +10 versions of the same character.
EDIT: The good thing is that you can feed your low merge units into high merge units without losing all your merges. So for example, if you have a 40+6 Hector and he has terrible IVs ("Old Hector"), then if you sacrifice him to a Hector with good IVs ("New Hector"), you will not start over. Your New Hector will not be a +1. He will be a +7 (and will have the all the skills of the Old Hector too). So you can change IVs at any point while you are building your way to +10. At the very worst, the only thing you will lose is all the SP you stockpiled. So if you have any skills to learn with your 9999 SP, might as well learn them before merging to change IVs.
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