Scouring Eternity is a casted spell. So it does not do other mechanics during this time. It’s just a repeated dance. The issue is players needed to get experienced, then it’ll be fine. Happens every tier.
The bomb persists into the cast and if he casts it right before going up for the channel it’s a pretty iffy timing.
If a bomb is up long enough for him to start casting and finish casting, the player who picked up the bomb was asleep and not collecting swirlies to go into the shadow realm. It’s easily possible to pick up a bomb before he starts casting and go into the shadow realm before the cast finishes. Which is why I say it is paramount for the people who get the bomb to rush into the safe bubble zone of darkness where they can collect their stacking haste buff until the bomb explodes because they are safe in there.
The overlap on Sarkareth (at least on Normal) is only made troublesome when people avoid picking up the bombs or when the ones that do want to be greedy with it before going down. Tell them to simply rush into the shadow realm, collect motes, and wait for the bomb to expire before coming up, the lost DPS by not hitting the boss until the last seconds of the bomb’s timer is worth not risking a raid wipe.
If you want to talk about overlaps that are painful, Heroic Zskarn. At one point late in the fight he will do his pushback (which applies a hefty DoT), a three-pillar tactical destruction which means you will be forced into a back sliver of the room, and his AoE debuffs which means if everyone goes to the same ‘safe’ sliver, they might just get instagibbed by overlapping debuffs. If there are any unlucky traps up in the middle when that happens, it’s almost guaranteed to be game over.
Again all true, again terrible design because it is pass fail and 1 persons error kills the raid. This is especially true given the COMBINATIONS, just like with Zskarn. It is the combination of these mechanics that is problematic. Also, great, multiple bad design decisions across 2 different bosses. I understand the fight, just because a fight is understandable doesn’t mean it’s good.
Sarkareth is a god awful fight because of how they designed multiple mechanics that work together that while individually may not be problematic together they are in this game. Anduin had the same problem, a single mechanic in the fight was not problematic, but together the mechanics were problematic. This is before discussing the absolute visual diarrhea the fight is with purple.
I mean, there are a lot of mechanics in the raid where, if done improperly, make the fight incredibly difficult or impossible.
Tanks that can’t taunt quickly? Won’t get past Rashok.
Entire group has tunnel vision? Won’t get past Assault.
People don’t know how to group up for dispels? Won’t get past Experiments.
People don’t know how to look at their debuffs? Won’t get past Amalgamation.
For Normal Sarkareth, all you need is a few people who know how to handle bombs and all of the sudden there aren’t any dangerous overlaps.
Let me put it this way. I appreciate what you’re trying to do. I understand, and I have understood everything that you are talking about during the encounter, I’ve tried helping others understand this, unfortunately that is easier said than done. That is why in large part I find this fight problematic.
You can disagree with me on the fight being problematic, that’s fine. I think it is, objectively I believe it is problematic.
Nah Blizzard loves one shot mechanics which can cause wipes not only in raids but also Dungeons 
But seriously tried helping a friend with their Neltharus key and I never wanna see Magmatusk again those lava waves are nasty 
Also the fact I’m stuck at 30yd range and kept on having people leave my range and then take a big chunk of damage was not helpful 
and I was looking forward to some special phase or secret true ending in the mythic raid, but… with what I saw the World First, this boss already nailed his coffin here 
.
Sylvanas had terrible cinematics but hers was her own mechanics, and the time she took to defeat her was impressive, here IDK, even the vanilla raid bosses had more fun.
And that doesn’t suit Blizzard to live in the past with modern WoW changing everything since Cataclysm.
It is literally going to go down as another Anduin. There is zero chance it is remembered fondly. Basically it will be “do you have the perfect pull? gg” or “mess up any mechanic? better luck next time”.