From the little I've read I was under the impression that Tantra was in the main a reaction against the dry Buddhist scholasticism of Nalanda and other monastic universities.
That's broadly correct, although I hold to the theory that proto-tantric elements in both Buddhism and Hinduism go back much further.
That said, it definitely was transgressive initially in terms of its sexual and caste ethics, even if "revolutionary" is kinda an anachronistic term for the first millennium. But it is also difficult to portray extramarital sex or women in authority as particularly radical today sure.
The thing is, most of the early tantric schools did not actually have women in authority. They had female or feminine divine figures, but if you look at the philosophical and ritual texts involved, many of them exalted these figures by contrast rather than comparison to human women. It's much like how the veneration of Mary in Catholicism doesn't necessarily translate to a higher position for women in Catholic societies relative to Protestant ones (quite the contrary, historically), because much of the basis for Marian veneration is Mary's dissimilarity to (most) other women. It's only in early modernity that we see the sexually transgressive elements of tantra actually translating into a positive reassessment of the role of women, and when it happens, it's partly due to Western influence.
