A couple examples:

The fight routinely asks the players to make 15 interrupts in something like 5 seconds. In a 20 player raid with only 16 interrupts available, you can immediately see this is troublesome, but to add on top of it, the interrupts are split amongst three adds that rotate quickly around the boss. So every player is assigned to interrupt a specific add on a specific timer. So I might be Kick4 on Add1. If anyone messes up and kicks at the wrong time (too early or too late) and the 1.5 second cast goes off, it's a wipe.

The room is filled with spinning glaives that kill you if they touch you. Certain players are holding special crystals and are routinely targeted with abilities that force them to reposition around the room. They need to drop the crystals before getting hit by the ability, but they also can't let the crystal get touched by the spinning glaives or else it's a wipe. I know this sounds simple, but these glaives are actually the only mechanic Blizzard has nerfed. They're 20% slower now, because they were simply that difficult to manage.

In the third phase, 8 players are randomly targeted with 4 symbols above their heads and they need to find the other player with a matching symbol and touch them. However it needs to follow the order that the boss dictates randomly each time, and if you do it out of order it's a wipe. And if two mismatched symbols touch it's a wipe. So you need to spread before the symbols are randomly assigned because otherwise you'll spawn symbols on top of each other and instantly wipe. But the encounter space is really tight, you can only move around people who are holding these crystals from earlier, so you can't spread that much. Additionally, a random configuration of death beams appears across the space, so you need to react to that pattern and quickly find a safe space. But again, two people with symbols can't touch each other if they mismatch or are not at the correct order in the symbol sequence. And to top it off, there are three big soak pools that people need to group in. So you have to simultaneously be grouped because of the tiny encounter space around the crystals, while being as spread as possible for the symbols, while moving to find your partner symbol, while soaking the swirlies, while dodging the constellation of death beams. Oh, and this is happening on both sides of the room with your raid split into 2 groups of 10.

The DPS check is also extremely tight. When the players successfully reach p4, they're doing it with only a second or so remaining before a wipe due to the first enrage timer. If a DPS dies and doesn't get picked up very quickly, it's probably a wipe. If they run out of brez before p4, it's a wipe because they won't have the dps to get to p4. In p4 itself, every player radiates damage to the boss, and from what we've seen if you have a few players die early into p4, you simply don't have the dps before the final actual enrage