I fail to see how someone with a connection to the root being slightly resistant to it means it's not on equal footing
If they were completely unable to resist it, that'd actually imply that it's on a higher level than the others connected to it.
And people born with a connection to the root having reality warping abilities while this doesn't also doesn't seem like a good reason for them not to scale. They do different things, one is meant to work on humans, and so it does, the other one is a more general use of the root's power.
The unified language also explicitly resonates with the root.
In any case, it has also shown to work on Shiki
"“That is not what you will do, Miss Shiki,” the mage says with an air of authority as if to emphasize a point. I should be at him by now, making him regret those words. But instead, they somehow hold me in place, preventing me from performing an action that something inside me tells me is purely wrong, even as my mind says everything about it is right."
"The mental block still makes moving difficult, but I manage to pump
strength into my legs to close the distance, raise my hand—
“Your sight is lost.”
I hear his voice for a split second, and in that vital moment, it echoes in my mind like an undeniable truth. The next thing I know, I see neither hide nor hair of Satsuki Kurogiri, and my knife swings only at empty air."
This next one is where her slight resistance comes from, and even then, she's affected
“Your Eyes will not avail you here.”
The statement echoes confidently above and below the chapel, and darkness rapidly begins to curtain everything. Through his words, I am deprived of even the faintest light in moments, and become surrounded by a world of darkness.
“Hmph. The first tongue has less effect on you than I expected,” he grumbles. “The connection of both of our spells to the spiral of origin grants you some measure of resistance perhaps. But in the end, the death you cling so close to remains unseen. As does everything else"
This is also despite his usage of it not being as good as it could be
"Godword is the only known one able to reproduce this universal language, and work his Art through the high speech. He communicates to all men, a channel through which that demiurgic force, that ultimate origin passes through. That his lack of talent for magehood prevents him from truly using it dangerously is a blessing for us.”
Here is a quote also explicitly saying it taps into and uses Akasha
“If the medium of words is used, yes. But the man is different. He speaks not truly to men and beast, but to the soul in them that still contains that last connection to something higher. There is always a part of us that is culled from that primeval spiral of origin, and when the high speech speaks to the soul, our fallen selves are compelled to obey. To deny it is to ontologically deny the very structure of reality, and thus impossible. An absolute language, that starts from statement and then proceeds to become truth. It is the ultimate form of hypnotism. He accesses the Akashic Record unconsciously, and through the high speech, taps into it to channel his will. It’s how he draws forth the memory, not from your mind, but from the Record of things past that reality still contains."