Hey, you will not get any argument from me that I find Vaishnavism a rather a stifling and fundamentalist religion(reminds me Christianity) and I don't agree with their doctrines that condemn or undermine other religions and sects and how they justify through their propoganda pamphlets - puranas. Hey, we Advatins constantly get attacked by the Vaishnavas, especially the Madhvas. They attack the Buddhists as demonic people.
That said, I just don't see any evidence for Shiavism and Shaktism i.e. tantra being any older than the early middle ages. I definitely don't see evidence for formalized tantra philosophy and scriptures. I think the evidence seems to show Vaishnavism is much older than Shaivism and is possibly the first Puranic sect to form within Hinduism. In the Vedic age Rudra does not seem to carry much importance, whereas Vishnu indeed does in the later Vedic times.
Hence I tend to see Shiavism and Shaktism as reactionary sects to Vaishnavism. I am open to correction if you can show me solid evidence.
I can sort of see why the Vaishnavas classify the Shaiva and Shakta sects as tamasic I mean come on communing with spirits, animal sacrifices, animism/shamanism, death rituals, ritual intake of wine, meat, ritual sex, fascinating for smearing oneself in ashes. This is certainly not what the Upanishads prescribe. At the same time I would classify Vaishnavism as rajasic, madcap devotional practices to Krishna, Rama, Hanuman, constant singing and dancing like mad people. It's all very woo-woo to me. This also is not what the Upanishads prescribe.
The true sattvic path of Hinduism is Raja yoga, chittashuddhi through meditation, morality, study and other kriyas. This is what the Upanishads prescribe.