Abilities =/= power. Hakumen is in a league of his own.
Again, Hakumen is not that powerful because of the unit.
In phase shift, Nine states that Hakumen was the ONLY ONE able to directly trade blows with the black beast.
I'm fully aware that Hakumen was the only one to trade blows with it, he is the strongest of the Six Heroes, but Terumi was not using the Dark Susano'o because of Nine's Mind Eater curse and it was Kazuma who was in control due to the split conciousness and who has no knowledge of the Dark Susano'o. Plus, Base Terumi who was under the Mind Eater Curse and had Kazuma at the helm of his mind to 100% Hakumen isn't a fair comparison.
And she knows how much powerful Terumi is.
Izanami stated to Terumi that she could erase him from existence with 0 effort, something that Terumi didn't denied.
Izanami was observing him. Once she stops, he's gone (which kinda proved useless considering Terumi's self-observation). If she could erase Terumi by herself without the need to observe him, why couldn't/didn't she do that with the other characters in the series if she wanted to?
While Rachel stated that Hakumen full power could beat Izanami. Even Terumi in centralFiction asked "Are you going to kill Izanami again, Haku?"
Not sure about this. Hakumen himself stated that he himself at full power alongside Jubei together would still have a lot of problems against Izanami. Hakumen never killed Izanami, as he did try in Chrono Phantasma with Time Killer, only to fail miserably and getting BFR'd.
He, with dark susanoo, has the same abilties as Hakumen. But not the same power