Main downside of the ancient set thing is that it'll bring back the old "fire on everything" builds, since you'll be able to stack elem again now that you don't need sets, and cindercoat/magefist is still superior to everything else basically. Maybe cold might be good with the redone gloves(they roll 5primaries including cold dmg, so like magefist) and the legendary gem, but cindercoat ressource reduction also made a lot of stuff more viable. It should make some old/unused skills potentially viable though, and should probably be great for maximizing low torment farm speed since builds with a bunch of proced legendaries and movespeed legendaries should be competitive/better than trying to maintain 6pc.
As long as it's not broken OP compared to sets, that's fine with me. The current values seem stupidly high though. Most sets increase damage by 1000% or so then they provide another effect like some sort of cooldown reduction or synergy between skills. And then you have to add specific legendaries that support the skills you us in a set. But if you have like 8000%increased damage, you can use the same skills, use the same legendaries that increase those skills, and just have way more damage on the exact same build, not even counting the other legendary slots that might add random stuff. Even assuming the bonus is additive with skill damage % rather than multiplicative like general sets, it still looks like a stupid amount.