The Brahman is stated to be beyond vision, speech and mind. In other words it is beyond intellect and words.
Na tatra chakshur-gacchati na vAg-gacchati no manah ||
To that place (of Brahman) the eyes don't go, the speech does not go
neither does the mind.
(Kena Upanishad 1.3)
But these philosophical doctrines are nothing but plays of words or attempts of describing the state of Ultimate Reality in terms of words which obviously isn't possible by any means.
Another thing is that Upanishads also state that those who think that Brahman is known to them actually do not have a clue.
YasyAmatam tasya matam matam yasya na veda sah |
vijAnatAm vigyAtama-vijAnatAm ||
To whom Brahman is certainly known to be unknown is the one who knows
it. And, who has the conviction that he has known Brahman does not
know what it is.
(Kena Upanishad 2.4)
The persons who are propounding such philosophical theories obviously have the convictions that they have known the Brahman and their doctrines are the correct descriptions of it. But the above verse itself shows that to such persons Brahman is still unknown.
A more explicit statement comes from Lord Shiva:
Ucchishtam sarva shAstrAni sarva-vidyA mukhe mukhe |
brahmano gyAnam-vyaktam chetanAmayam ||
All scriptures are studied by many people and hence they have become
Ucchista and so are the numerous forms of knowledge; The Knowledge of Brahman, on the other hand, has never become Ucchista till date
as no one has ever been able to express that state of consciousness.
(GyAna Samkalani Tantram Verse 52)
Ucchista actually means leavings or remnants of food. The left overs after someone has ate.
All the scriptures have attained that stage because so many mouths have read them. But the state of Brahman is inexplicable.
Yet another verse is the following which states that no such philosophical theories are capable of depicting the Highest State accurately.
Advaitam Kecidicchanti Dvaitam icchanti ChApare |
Mama Tatvam Na
JAnanti DvaitAdvaita Vivarjitam ||
Some prefer Advaita (monism), some others prefer Dvaita (dualism). But
my true nature (or the nature of the Brahman), which is devoid of
Dvaita and Advaita both (DvaitAdvaita Vivarjitam) is not known to
either of them.
(KulArnava Tantram 1.110)
So, obviously all such theories must be inadequate. And your claim is supported by scriptures.