I'm sure others have already stated this sort of thing, but I imagine a lot more of us are encountering it now with additional sands of time flooding in:
Portions of 2nd Coil of Bahamut: Turn 2, and Turn 3 change in infuriating ways when you move above a certain DPS point. My group has consistently cleared 2nd Coil 1~3 for months, and suddenly we are wiping again and taking WAY longer than we should to clear them. It's to the point where people want to ragequit coil entirely, which really sucks.
For instance, in 2nd Coil Turn 2, when the boss reaches certain HP% points, additional mobs spawn, and when these spawn, it causes an interruption to the bosses skill use rotation. Because of this interruption, the boss will often fail to use voice again in time for us to petrify another Renaud (or re-petrify one), causing a wipe. Currently with my group, this out-of-sync issue tends to happen just after the final add has spawned and died, so it's a long 8ish minutes til we wipe each time. We have combated this FLAW by intentionally turning our backs and not DPSing the boss until she uses shriek at certain HP percentages, but this does not feel intentional, nor does it fit the spirit of what a boss fight is about in my opinion.
In 2nd Coil: Turn 3, higher DPS causes towers to skip ahead to the 3 tower config sooner, which causes issues syncing up the AoE dmg tower with Allagan field. Tower AoE hits Allagan Field person, we wipe. For this we eventually worked out a different tower order and cleared after much longer than a usual run.
BEFORE we had better gear, the battles both flowed perfectly. I find it asinine that we should have to hold back DPS or wear inferior gear to maintain proper flow through these fights. Unless SE is going to drastically change the fights when echo appears in 2nd coil, I do not see echo helping much.
I believe this sort of issue is just going to keep happening, given the way gear progression is working. There's going to a "not geared enough" point, a "geared just right" point, and an "over-geared" point in raids moving forward. I just hope the developers are testing out these battles in all 3 possible sets of gear and looking at what happens to the mechanics of the fights.
For as long as I've played Final Fantasy games, and RPGs in general, one of the funnest aspects has always been over-leveling and over-gearing your characters and then utterly destroying everything in the game. While I understand the need for balance in an MMO, I do think past accomplishments should continue to make redoing content easier over time!
Edit: To clarify, WE CAN CLEAR THESE STILL, I'm just saying it feels like poor design to have to basically STAND THERE until the mechanics catch up. There are even times when standing there still wasn't enough on 2nd Coil Turn 2. Honestly, it felt like it went better when we just had a couple of people DIE so the DPS lowered. That is plain dumb.