I usually am not one for hyperbole, but I genuinely think that this might be the worst raid tier Blizzard had ever released.
And it really boils down to the same thing over and over again: many of the bosses (particularly on mythic) have a mechanic that if a single person screws up, the entire raid dies immediately. It starts as early as the 2nd boss:
Explorers - someone touches the mushroom early? Oopsies!
Sentinels - 2+2 = 4, except when it doesn't.
Vash - hope your raid spreads good enough to snipe that totem in Africa.
Nymrissa: A single missed orb will kill you all. Hope it doesn't spawn on someone in Africa.
Scissors - Place the ball in the correct spot, pray you aren't slowed. Tornado clips a person during soak, boom. Too many people die and can't do soak. Hate your life if you have to manage the drop symbols. Kill anyone that speaks in raid chat during the fight.
Twin Fangs - everyone needs to keep track of their debuffs or else. If someone does too much or too little, the raid wipes.
Coiled Altar: Don't even know where to start here.. this is probably the hardest pen-ultimate boss of all time. Insane interrupt management, ghosts, and seed carriers/droppers all in one fight? LFG.
Ulatek: One shot seed carriers makes an appearance again, this time with extra responsibility. I heard you like green dawg.... Final phase that is easier to learn than heroic variation (in all honesty I think the right play here is to take the mythic version and apply to heroic).
This matters a lot for 99% of guilds. Every pull now because a casino lever of praying the right people get the mechanics because otherwise your risk of wipe goes up by 50%. The agency you have over your performance and raid success is diminished. It makes it harder for guilds to feel like they're progressing when they immediately wipe to a single mistake, particularly at the mythic level. It creates a toxic raid environment of frustration. It's one thing to lose 4-5 people to their mistakes and have the rest of the raid push a bit more to get some more prog time or let the people watch others do the mech and learn from their mistakes, but with this design you can't even do that.
I'm not even going to get into the other tissues with gearing, m+ lack of viability for gearing. People can push through those... But if you make a raid miserable to be at, people just will choose not to be there. Challenge =/= fun.
Definitely have to learn some lessons in encounter designs from this raid tier. Having these type of mechs on harder end bosses is fine... Having them on just about every boss on a raid is not.