The raids are definitely overtuned in the sense that they are harder than the tiers of the last several expansions. The kill numbers for mythic sire months after his release, worldwide, are still under 10k. Those kind of numbers are almost Sunwell levels at this point and time.
Mythic is supposed to be hard, but accessible. If (arguably) the best raiding guild in the world is saying the content is more challenging than expected, that’s a very strong argument to try and counter. Blizzard seemingly agrees that the number of attempts : kills is skewed (overtuned) by the weekly nerfs they seem to be instituting. Sun King and Hungering a few weeks back, Innerva two weeks ago, Council and heroic Sire last week.