People don’t realize how easy it is to get into raiding in this game

[Valence]

Okay so I am not sure it's exactly what you are looking for, and it's only one fight, but Halicarnasus in Haukke Hard has issues due to ilvl/potency bloats, but this is mostly an issue while sync'd, she will summon an add, and you need to get the add to 1 HP as quickly as possible, after a period, she will eat the add and use an AoE based on how much HP the add had left, if you burst down her HP too quickly, she will summon the add, immediately eat it because you hit the health gate on her next mechanic and she wants to move things along, and wipe the party with a full powered AoE, so you either need to consciously hold DPS as soon as she starts summoning the add until it is targetable, which is unintuitive, or for the Tank to use LB1, which will be enough mitigation to survive the full power AoE, but unfortunately not all Tanks are aware that's even an optimal choice there, and I don't blame anyone for not knowing/remembering.

That said Haukke Hard is very unique about this, I have experienced wonkyness while Light Farming for Zodiac relics, like Shiva randomly starting phase transition at 80% HP and I do not understand what makes her do that especially because she only did that a portion of the time, but that was never a big issue, and I can always go bully Bahamut in T12 instead.

But that was already part of the design of the boss back then. I see what you mean in that now the margin is tighter since you can't afford to damage her even less than before, but when this boss was current people still had to stop DPSing her in order to focus on the adds.
Which isn't very different from what Ramuh's arbiters (the first set) have been like as well since day one.
I'll not say that's the best kind of add design though, because frankly that's super binary (everybody dies feels super arbitrary). A smarter design could have for example added a very nasty ice spikes on the boss during the adds so that people literally dent themselves over the boss, or anything.

[VeyaAkemi]

But that was already part of the design of the boss back then. I see what you mean in that now the margin is tighter since you can't afford to damage her even less than before, but when this boss was current people still had to stop DPSing her in order to focus on the adds.

The difference is that now the gap is large enough that a competent team can push her down to "eat and nuke" threshold between her being locked into the mechanic and the summoning animation finishing so the add is targetable, the problem isn't "people are ignoring the add and focusing on her", the problem is "the game decided the mechanic was over before it started".