People are saying "too much time to react", but some of the mechanics go off in 5 or less seconds, which is NOT a lot of time. I remember getting annoyed in P1S (the first ever Savage I attempted at level) because the Fourfold chains mechanic gives you ~3.
Ivalice mechanics gave you more time than this, and that was touted as the hardest raid series of all time.
I think a lot of high end players don't realize they're high end players and don't understand that what challenges them will devastate others, especially since the general playerbase isn't in Savage 660 gear. TG Cid's mechanics come out at a similar rate to these 24 mans - and did before the nerfs, too - yet no one said that raid was easy. At this point, I believe if Orbonne was released today, people would be insisting it was too easy. "Mustadio's telegraphs are easy and give you 10+ seconds to react and he attacks slowly." "Agrius just has you use a battle reaction command, boring and my parse!!" "Cid's attacks are slow and the telegraphs are easy." "Ultima just uses some recycled attacks from the last raid. The devs are so lazy!!"
I agree that something that forces wipes isn't difficult, it's badly designed. I saw an analysis of Dark Souls vs Remnant: From the Ashes (also known as "Dark Souls with guns") that praised the latter while ridiculing the former. The nutshell version is: Dark Souls is cheap and kills you over and over, Remnant's fights are fair and you CAN beat the boss the first go if you're paying attention. It's the difference between a game designed to frustrate the player, which some players like and mistakenly call "challenge", and a game with good design where the encounters are fair and if you die, it's on you; actual challenge.
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As for this 24 man:
Casuals dislike it because it hits too hard, the tells are a bit confusing, especially on the really busy fights where you can't see anything through the spell effects (the last boss 3x charge it's REALLY hard to see which arm is lit up) and the mechanics come at you hard and fast and crazy, with the safe thing to do not always readily apparent. It's too visually busy and, especially the last fight, there's too much going on all at once. The mechanics don't hit as hard as a Savage (and you have more people to drop and still push the fight), but they come out at a pace similar to a Savage.
Hardcores dislike it because it isn't at Extreme level and isn't wiping parties until people figure out the mechanics.
When the metric is "there should be full party wipes before people clear fights", that's a Savage thing.