If latency and reaction time are low, if you (as healer) get distracted because others mess up and send out some shield after the cast has alreday started it can get close, especially if you are used to this kind of abilities not being on the GCD - as is the case with almost all of them.
I do play other classes and it alwas irritates me when SG does not activate when I want to use it because it is on GCD.
Also: If you use SG too late into the cast he will just throw anyway.
Why are you getting to "a million stacks" then? Fail strategy is fail.
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Why do you *need* to guise sawblades though? Does it cause raid wipes? No. Does it limit safe spots for the raid? Not when the room is so big. Does it kill you even if you clip it? No.
It's not about the "can", it's about the "why".
"My successes are my own, but my failures are due to extremist leftist liberals" - Party of Personal Responsibility
Prediction for the future
While the rings of fire are up on heroic Siegecrafter sawblades are a pain for healers.
It's not like whoever gets sawblades can't just run slightly to the left/right/whatever, such a big deal. Plenty of safe spots to stand for rings of fire.
And if you don't have any priests or classes who can drop off the threat table for sawblades, are you going to be helpless? No, just do the damn fight as its supposed to, run sawblades to the side.
"My successes are my own, but my failures are due to extremist leftist liberals" - Party of Personal Responsibility
Prediction for the future
The "Why" is pretty obvious. Have you even done the fight? I guess depending on your belt order, your strategy might be less punishing, but Guising sawblades eliminates movement at the cost of a GCD. For 99% of players, this tradeoff is worth it. I can't count how many times I've started moving to take the warlock portal for belts and either get targeted with Sawblade while I'm running to the portal, or mid-flight to the pipe. Guise prevents bad sawblades both where the raid is standing, and on the pipe.
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Yeah, if there is one second on the cast, he will throw, but with WAs and bossmods, this shouldn't ever be an issue. Your reaction time just needs to be better.
You can guise Siegecrafter's laser as well. It'll fixate onto somebody else without telling them. If it wipes the raid, "wow that mechanic is really buggy lately."
(Protip: Only do this in Flex.)
On my priest I have used it for every boss in regular and up to Nazgrim on heroic, it's drops you out of combat as long as the clone of you dies it confuses the boss, just get away from it and you live. I've even used it in the WoD Alpha/Beta so no nerfs there. It's not even temperamental if you know how to use it. Best Talent in that branch imo.