I'm willing to argue that the biggest reason a lot of people suck at the game is because of the following:

Everything important to pay attention to is miles away from your peripheral vision

Debuffs and self buffs by default are tiny icons, miles away from your peripheral vision at the tippy top of your screen, miles away from your peripheral visio. Target status, where you manage your DoTs etc. are also all the way up there. Target Progress bar, Enemy list, everything that you need to pay attention to to solve fights. Meanwhile your *very important* hotbar is in antarctica at the bottom of the screen. Not to mention mechanics that are usually happening in the center of your screen demand your attention as well.

Even high-end players move them to the center of their screen, then increase the size of by 200% to be able to reliably react to them. Sure they're right there. But is any casual player ever going to really notice beyond even if debuff kills you? Even if it literally tells you how to solve the mechanic? Absolutely not. And even if it does kill them, they will most likely not be able to solve it until taught via word of mouth.

People laugh and point at SGEs that dont use kardia, DNCs that don't dance partner, tanks that don't mit, and RPRs that don't use Shadow of Death, etc. Meanwhile the game makes it exponentially harder for people to intuitively monitor these things, and to make sure that they're not forgetting to refresh them. I'd be willing to bet that sprout healers have a much harder time thanks to the fact that the party list is in the upper left corner of the screen. This makes it a LOT harder to heal members if you use M&K and click to target. Also esuna debuffs. Basic things such as making all dots, or self buffs you need to maintain a part of the job gauge or moving the target info right above your hotbar would vastly improve new player experience.

Most of this game's content is extremely predictable, scripted, and easy to solve. But I don't blame newer players for assuming that the game is twitch based, unpredictable, and hard to adjust to when the UI has your attention constantly pulled in 10 different directions while expecting you to solve mechanics.

You can argue that the HUD is customizable, so these changes arent needed. Meanwhile Stormblood added job gauges. Hell they're improving the originally just as awful Crystal Conflict UI next patch, even though it's always been customizable.