I think KISS goes down a list and may do one thing before another. Holy's/downs pretty much will immediately do whatever right when the conditions are met and can do it whether you're running a mac or not.
Like for some toons, I like them to always have an illusion up and then shrink and it's set up in melee to do that all the time, unless in certain zones or invised.
On my enchanter, I want him to cast darkened rune if my tank ever falls to 40% health.... immediately if not sooner. I could not get that to happen with KISS, so I moved it to melee. When I had it in the heal section of KISS, it didn't work at all. In the DPS section, it sometimes tried to nuke instead of rune or even mez. With the downshit, my tank drops to 40% or below, chanter will /stop cast and rune and then back to KISS which will probably AE mez.