They're not hard, they're just more fun. Vanguard 2 boss comes to mind with the DDR AoEs that feel hectic at first, but once you run it a couple times you realize it's just Ultima EX's line spam with a small bait AoE every few seconds. People complaining simply don't want to put effort into playing the game. I've had a LOT of spam healers in dungeons and have had to stop suggesting they use their AoE because of how many of them get all defensive and trigger-happy with the votekick feature if they're queuing with a bum chum. Similarly, healing some tanks, particularly DRKs when they don't understand the kit is just as painful as it's always been.

The short answer is always just "people are lazy."