The common issues I see stem from people not recognizing "non-standard" tells. If there's not a circle/square AOE on the ground, they don't know it's an AOE. In these situations during normal content, the AOE only appears for a moment, and at that point it's too late because they aren't looking for boss position/cast name tells. Coupled with that, some of the mechanics do have a little more strict execution - nothing more difficult that like, Aglaia-tier difficulty, but still more strict than Endwalker was. Adding these two together, you get a situation where some people don't understand how they're messing up/blame lag, and are struggling to execute the ones that they DO see, so they eat a ton of vulns or other debuffs. They usually are the same people that only play with trusts, so that death is a full wipe instead of a raise/just keep killing the boss situation.

It's a combination of a few things - Endwalker lowering the skill floor below the basement, no ingame tutorial on how these low-intermediate mechanics work (to set the expectation that this is how the game is); and as much as I hate to say it, a skill issue.