Context: I'm a veteran player who has been raiding Savage/Ultimate for years and have leveled several alts by hand.
One of the greatest failings of FFXIV in its current state is that it doen't teach you how to play. Sure, when you start the game it teaches you how to move your character, the myriad steps involved in doing a basic fetch quest, how to use aetherytes, and more through never-ending "Active Help" text dumps that look like ads just begging for you to close them. But while the game has evolved significantly since its release nearly 11 years ago, most of the early game has not.
Power creep reigns supreme through entire expansions, and is especially apparent in those ancient solo duties whose only requirement for success nowadays is "have a consciousness". Players never learn how to fail until they're eating dirt over and over to newer content and they understandably get frustrated.
Watching the boss's castbar is integral to modern FFXIV fight design, and yet its default position is this tiny little toothpick-sized speck tucked away in the upper-right corner of the target bar. THAT'S why we have people say mechanics are "seemingly random" or "come out of nowhere" -- because they have no idea they should be looking at the cast bar for information, they might not even know it's there. Think the final boss of the lv89 dungeon -- he can cast half-room cleaves called Left Firaga and Right Firaga, where Left and Right indicate what hand he's gonna cast the cleave from. The cast bar is the only indication this attack is coming until it's too late to dodge it intentionally. It's an incredibly basic mechanic, but if you didn't know to look at the cast bar, you'd be thinking "Wtf? How was I supposed to know?"
The game makes almost no effort to teach you how to play your job beyond the most basic of basics. I've been in duties with 0 DPS healers, tanks who don't mitigate, Black Mages who spam Blizzard -- hell, I was in the lv95 dungeon around a week ago and the Red Mage with us didn't press a single AOE attack the entire 23-minute dungeon, dualcasted Jolt more often than not, and never touched half of their oGCDs. Their search info informed me they had somehow gotten all the way to level 98. Likewise, a few years back I watched a friend attempt Pagl'than (level 80) with Duty Support. On the first boss, he didn't seem to realize that standing in the lightning pulses gave him vuln stacks, and due to tank privilege he wasn't in any immediate danger of dying so he didn't move out of them (maybe didn't even realize he was getting hit by an avoidable environment hazard?). He accumulated 8 vulns. When the tankbuster hit him shortly thereafter, he went down like a sack of potatoes, and had no idea why.
The game seriously needs to start expecting more than the barest of bare minimums from players way, WAY earlier, so they can learn iteratively throughout the entire 1-100 leveling experience.