The difficulty jump from normal heavensward to stormblood when concerning the primals in the game was tremendous compared to the old content when it launched.
The first time I did Tsuku normal, the raid wiped over 3 times until I took control of the group as the tank and guided them through it. After that, they succeeded. And even then there was still a ton of deaths. There still is for those coming into it.
Even then, comparing sync to unsynced is disingenuous, even on its face concerning difficulty. Primarily because with enough ilevel and damage, you skip mechanics. Suzaku Ex and her tankbuster/sweep knockback/instant death mechanic from auto attacks due to the debuff is a prime example of this.
Unsynced, you can skip that entirely, lowering difficulty significantly. It is, even at face value, not the same fight when considering that. And good groups mount farming or mat farming know how to force the skips unsynced easily.
Byakko Ex is also an example. A group who knows to stop dpsing him at a fixed percentage of his health unsynced can trigger him into an animation cycle where they can then force the skip of the tornado sequence and also a large part of his mechanics.
So, imo, unsynced is far, far, far easier than running synced.
One can also say final coil of bahamut unsynced does not compare even relatively well to the difficulty of it if it is ran synced. You cannot skip mechanics synced. Just not possible. And That raid, twintania especially back in the day, gave folks no end of fits.
% shift over night because of fear the ban.