Any healer who has done SCOB T4, FCOB T4, Alexander 3 (savage) and/or King Thordan (Extreme) must have noticed the fact that healers keep building enmity through healing during phases where the boss cannot be targeted. The fact it cannot be targeted means that the tanks can't do anything about it either. This hasn't posed much of a problem yet, as current content does not instantly murder you when the phase changes. But for the SCOB and FCOb veterans among us can surely remember the Raven's descent and Ahk Morn being the first thing that will happen as the boss can be targeted again. For those who haven't done those back when it was relevant content: It usually meant instant death if you weren't a tank and on top of the enmity list: https://www.youtube.com/watch?v=neT3uh0CRwU
Normally this was never really an issue here as long the White Mage use Shroud of Saints frequently to dump enmity. In the video, however, the White Mage forgot to use Shroud once (normally twice) and got the entire group murdered - We dubbed it the Sephiroth dive.
DPS jobs obviously do not suffer from this kind of problem. Unless, of course, murdered because a healer happened to be near him/her and on top of the enmity list. Among the healers Astrologians have the biggest issue, Scholars "moderate" problems and White Mage the least.
Scholars don't have any enmity modifiers, but they are in a special situation here. A part of the healing enmity is dumped on the fairy and they're less likely to be the one AoE healing of the healing pair.
White Mages obviously have the least issues with it: They can almost use it whenever they really want to and dumps your enmity while you use Shroud of Saints for MP.
Astrologians Luminiferous Aether (LA) works a lot like Quelling Strikes: It prevents a chunk of Enmity build up. You could argue that an Astrologian could save LA for certain heavy AoE healing moments. And this does happen to align perfectly if you use it on Dragon's Rage in King Thordan. But this isn't always the case and I've personally gotten into situations where I've on top of the enmity list depending on RNG and the group: Whether we kill one knight before Holiest of Holy or not. And god, King Thordan has a long phase where the boss cannot be targeted and requires quite a bit of healing.
Interestingly enough, healers aren't the only ones "suffering" from the enmity build up. Tanks can't do anything about it until the boss can be targeted again. While tanks can build up some extra enmity before the boss struts off like haters gonna hate, this comes at the cost of their individual performance. In particular Paladins take the biggest dent in their performance here, considering they have the lowest enmity combo potency among the tanks. The other issue for tanks would be the amount of time they are given to build up enmity before the boss struts off and the duration of the next transition where the boss is untargetable. Depending on both durations, the build up enmity may not even be sufficient to last till the phase after.
Bringing a Ninja can serve as a band-aid fix for any enmity issues the group would have. Or simply have the warrior open the fight with unchained for some head start enmity build up. But isn't this "gimmick" worth considering as a design error, rather than a weakness of a job? If tanks could actually do -something- about it, sure. But this is simply an annoyance for certain jobs, players with a certain (dis)ability and certain group compositions and maybe even a lethal one for future content.
tldr; Prevent enmity building on bosses through healing while it cannot be targeted or is invulnerable.