Who is adjacent and who is chaining seems to make quite the difference. If you are attacking a lancer and you have a mage adjacent and chaining into your archer, it seems that an adjacent archer chained into a mage does more damage. So I think it's best if at possible to use someone instead of a healer or mage to be adjacent and use the supporting units in chains. Not only that the adjacent units effect the proc. Skills like Megasword(1), Megashot(1), thunderblade, etc. only seem to proc when the unit attacking is the adjacent initiator (you also notice that their portrait isn't blinking like the chained portraits on the windup of the attack). But some skills proc no matter the distance like the splash damage dark spell from Zerro and aoes like 2nd job Grace's Megashot (area), but attacks like Megashot(area) center around the user, so it can whiff and hit nothing if she is too far away. Not only that but if you have two attack spells, they can both proc, like an easy way to do damage with Grace is set Megashot(1) once you unlock her second job, that when either by chance or Power Circle she procs both Megashot(1) and Megashot(area) ONLY IF she is the attacker, not the chainer, since if she is only part of the chain she does just Megashot(area) and whatever buffs she has.
This was important for when I was farming Pudding Hunt 2 (which had physical resistant enemies) because the adjacent initiators needed to be Amazora and Lizard King who had Flameblade and Thunderblade, while Zero my mage could hit with an element from any range as long as she chained.