Greg, don't misunderstand me, I'm not attacking tathagatagarbha teachings but actually subscribe to them as a follower of the Buddha-mind School (i.e. Zen). Nevertheless, the concepts of Buddha-mind, original purity, etc. are literally substantialist teachings. Here is something to ponder in that regard (Platform Sutra, ch. 8, tr. by John R. McRae):
[Xingchang] said, “Your disciple has been reading the Nirvana Sutra constantly, but I do not understand its doctrines of permanence and impermanence. I beg Your Reverence, in your compassion, to explain these for me briefly.”
The master said, “That which is impermanent is the buddha-nature. That which has permanence is all the good and evil dharmas and the mind of discrimination.”
[Xingchang] said, “What Your Reverence has said is quite different from the text of the sutra. ... The sutra teaches that the buddha-nature is permanent, but Your Reverence says it is impermanent. [The sutra says that] the good and evil dharmas and the mind of bodhi are all impermanent, but Your Reverence says they are permanent. This difference has made this student even more confused!”
The master said, “Do you understand? If the buddha-nature were permanent, then no matter what good and evil dharmas one explained, not a single person throughout the entire eon would generate bodhicitta. Therefore, I preach that it is impermanent. This is precisely the Way of true permanence preached by the Buddha. Furthermore, if all the dharmas were impermanent, then everything would have its own self-nature that would experience birth and death, and those true and permanent natures would not be omnipresent. Therefore, I preach that they are permanent, which is precisely the true doctrine of impermanence preached by the Buddha. Because ordinary people and heretics are attached to false permanence and those of the two vehicles consider permanence to be impermanence, together forming the eight confusions, the Buddha in the authoritative teaching of the Nirvana [Sutra] destroyed their prejudices and revealed his explanation of true permanence, the true bliss, the true self, and true purity. You are now relying on the words but going against the meaning. With an annihilationist impermanence and a deterministic permanence, you have misunderstood the Buddha’s last words.”
As for the mystical and esoteric, they might have some value in being cryptic, but I find that the case is rather that integrating a contradictory teaching requires only the right amount of explanation. Like, the sutra says one thing but actually it means another thing. This is creative exegesis.
"While teachers of the middle way, mind only, transcendent wisdom, mantra, and other schools may have their own assertions, the fulfillment of those intentions is the same. There is not a single thing that is not contained within mind."
(Gampopa to Düsum Khyenpa, in "The First Karmapa", KTD Pub, p254)
“If you recognize the world of appearance and existence as the mind, realize the mind itself as empty, and have no grasping at the superiority of your realizations — this is the ultimate view."
(Chegom Dzongpa, in "The Book of Kadam", Wisdom Pub, p609)