There are times when I play this game where I get incredibly frustrated with mechanics not working as intended. I see people die and/or hindered due to no real fault of their own.
To be clear, I am not talking about lag/latency, although that is a topic worthy of its own discussion. I am referring to mechanics not working even if you have done something way ahead of time.
Almost all of these fights are salvageable even with these odd glitches, but it's still super annoying when it happens.
Here are a few examples I can think of off the top of my head:
Garuda (all modes): whenever she jumps, there is a chance that someone in your party won't see her when she lands again. It's a known bug and has been happening since people were working on Garuda hard (and possibly story mode?). She will reappear after she jumps/lands a second time.
Garuda (extreme): occasionally, the spiny plume bubble will not work as intended. A party member will be in the bubble way ahead of time (and the entire group will SEE that person in the bubble way ahead of time, just as long as anyone else), yet they die as if they took unmitigated damage -- basically, the game never registered that they were in the bubble despite the fact that they were. At first, my group thought this 'glitch' was more likely to happen if a scholar dropped soil on top of the bubble, but then it happened when there was no soil on the ground, killing that theory.
Titan (all): people have been complaining about getting hit by landslide due to latency for a long time. That is not what I am referring to. What is really frustrating is if you get hit by it when you were never in the trajectory, neither before nor after it triggers. I've personally watched people get hit off who I never saw in the line of it. My guess is the radius is slightly higher than the visual cue, or character positioning is not 100% accurate in this fight.
Titan (extreme only, I think): sometimes, landslide and bombs will not trigger at the time they usually do, or Titan will completely skip landslide in his rotation. This is survivable, but just plain weird. I'm not talking about the heart phase, where I know his abilities can trigger at odd times if the heart is almost dead.
Turn 5 (Twintania): on rare occasions, icons marking conflag and fireball will not appear on a player's screen -- or on really rare occasions, anyone's screen. There was one time my group was fighting Twin, many weeks after we conquered conflag phase (it's very predictable). After the first fireball, we all moved and waited for the conflag icon that was sure to come next... but... it never came. We all stood there dumbfounded, and just as one of us said in voicechat 'where is conflag?', it dropped on a random person and trapped their group of people. Upon discussion, not one of us saw it. As far as I have experienced, this issue (icons not appearing) does not happen in any other part of the fight -- just conflags.
Can you think of any more examples? These are just the ones that I could think of off the top of my head.
EDIT: I've added some things mentioned by others. Also, to be clear... in case someone didn't actually read the title: I am 100% okay with the difficulty of the fights. I just want the mechanics to be less buggy.
Hate list bug (mentioned scenarios: Ultima + Bone Dragon in CT): on some fights, the monster will attack people who are NOT listed as top of the hate list. This makes it very difficult to 'watch your hate' if you can't see the aggro bar properly. I'll put this as a 'possible' bug because it may be part of the boss mechanics to attack a random person.