According to wowprogress, only ~7% of those who have defeated the first boss has managed to down garrosh- out of those, only half have managed to down H Immersous.
Consider that 2/13 H was the norm for most guilds after downing Lei Shen (Jin'rokh and Ji-kun) within the first week, AND that most of the world first guilds ONLY ran into thier first block at H Dark Animus (Whereas for those who progressed heroics over time, Council was the first block, which was generally 6/13, close to half the raid downed)
The main thing with Throne was that there were several early blocks- Horridon N, Tortos N, and then Durumu N. Once you had those 3 downed, most guilds acquired 11/12 N very quickly, with Lei Shen being the next block. After Lei Shen was downed, guilds would have Jin down as early as the next reset, if not within a month. Ji-kun comes along quickly after, and then difficulty begins to slowly ramp up.
The main thing with Throne was that difficulty with boss progression was all over the place- You had really hard early bosses, and faceroll later bosses. Blizz has actually stated that they felt the blocks in places where progression should be smooth was something they didnt like and would attempt to have much more smooth progression the next tier, which is why you have the first half being easy, and then ever increasing difficulty.
Finally, you have to realize YOUR progression level. If you finished up T15 at, say, 6/13 H, you are a moderate heroic raider, and SoO N will be seeing the fight once and adapting to it quickly. You should be out of normals within the first month, and will be working on heroics shortly thereafter. IF you managed to get 7/14 H done in the same time as it took you to complete normals, then yes, SoO H is greatly undertuned in comparision to ToT H. However, if, within a month after completing N T16, you are at 3/14 H, then SoO is designed at the same difficulty as ToT was.
TL;DR, World first guilds steamrolling through heroics at same pace as they steamrolled T15 is nothing new. Heroic progression guilds steamrolling through Normal T16 like they did normal T15 is nothing new. If Heroic Progression guilds steamroll through H T16 at a similar pace that they steamrolled though Normal T16, thats a failure of raid.
If, within 2 months, the %age of raiding guilds at 5/14 H is greater then 10%, then yes, SoO is significantly easier then ToT. Atm though, <10% of guilds have even downed Garrosh, <5% of guilds have downed the first boss on heroic, and only a mere .29% have downed more then 4/14 H.