I played destro in MoP and it was hella fun and engaging since it had a definite meta playstyle of trying to fit as many CB's into procs as much as possible which i enjoyed.
There was something about having all your trinket procs align when you had 4 shards then popping dark soul and just giggling as you literally unleashed hell on the target!!
Builders hit for literally fuck all, but i didn't care as they were there to build and fuel the CB goodness, i didn't even mind using RoF on single target as it gave you an extra button for movement as well as a sustained source or emberbits to further counter act not being able to generate on the move once your conlflag charges were gone.
And then there was shadowburn, oh how i miss that spell, the change to its current form in legion was the main thing that drove me away from destro, for me that spell is more iconic to destro than CB ever was, after all we had it way before CB was a thing and to neuter it down to is piss poor weak usless form was just the final kick in the dick for me.
The spec just worked in MoP and i can't help but think it was the morons who couldn't plan ahead for things like movement or just blindly spamming CB's without any real thought for waiting for procs etc that got i changed into the abortion that it was in WoD with incineratebolt, but i digress and going off topic a little here.
Destro mastery is certainly one of the elephants in the room IMO i mean why would you implement a mechanic that actually makes it harder as a dev to balance a spec, while pissing the playerbase off with more RNG?? Not only that but the whole point of Chaos Bolts is they crit all the time, so mastery is basically Crit 2.0 for chaos bolts??? Why the fuck would you do that??? It's reintroducing the complete opposite of what the spell is supposed to represent.
With pvp templates now being a thing there is zero excuse for not having a super sayam chaos bolt, take the cost down to 1 Soul Shard, reduce filler/builder damage, buff CB to similar modifiers as MoP and make the T21 4p Baseline........you know like it used to be (how lazy a 4p is that!!) and fixed.................
Wont be done tho, cause the other elephant in the room thats actually the real reason destro is playing like it is, is the artifact. There are only so many things that you can do to CB in terms of modification via the artifact talents, and there are way too many points to be spent so they have to divide them up through the other spells, every artifact ability that buffs damage elsewhere equals a necessary reduction to other spells i.e CB for balance reasons so we arrive at why buffing CB can only go so far so that the other shit the artifact provides still feels "meaningful"
TL: DR Destro can be fixed but the only way really that we will get a MoP style spec back (the best iteration of destro to date IMO) is getting rid of the artifact so there is no necessity to spread the damage out over so many different spells.