Strictly talking <30% and belt legendary..
I would think our worse stat is either mastery or versatility. Haste has almost identical drawbacks, FB, PF and Pyro in that all those are instant. Further raid pops hero and haste just lost value. Scorch and Pyro are still incurring the GCD of 1 sec, haste doesn't change that. In reality it is probably a wash sub 30% with belt because the crit heavy also has haste, while the haste heavy also has crit.
Are you saying it's not worth using combust burn phase sub 30% and therefore no need to bank 2 FB/ 2 PF for that?
Otherwise all was saying is when no HU, force it so scorch gives a HS.
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Also wanted to comment on....
While I don't disagree with what you are saying, Blizzard does. They have stated that balance is looking at both Mythic+ and Raiding.
Other thing Blizzard has insisted on keeping is this notion that pure DPS classes should have niches.
Toss in their favoring melee this expac... oh well