Ths is a novice’s attempt to answer about why the practice of some deities:tArA,chinnamastA(to a small extent vajrayoginI too),vilAsinI also is soiterologically relevant:
[Edit:Please do not take this as some word of shAstra or AchAryavAkya or something,I am not any guru,I am just thinking my thoughts out loud after asking my question and trying to process what I got]
An example of animus Buddhists displayed to Hindu systems can be seen about. This ritual consumption of brahmin flesh(a huge NO in any Hindu system that will ever exist) stems from two sources:(1)A kApAlika background is suggestible,as noted by the offering of brahmapuShpa to gain power as recorded by the author(I am not saying that that system existed in toto like that at Adi Shankara’s time but it transmits some relevant information about a cult with similar practices) and (2)actual dvesha for brAhmaNas,the gurus and systemizers of a lot of the native systems in the first place from where they took our mantras from:Hence these deities under the influence of the Bauddha transmitters took on Bauddha dresses in various aspects(like Rishi,etc)
With deities worshipped by such people,how is it that our people gain mukti?Taking leads from this article suggested by the author of the Kamakotimandali blog suggests that without a proper understanding of the tattvic scheme,such Bauddha upAsakas in the 36 tattva scheme would attain at best Buddhi tattva,mahAdevIing the chintAmaNI she is, granting the sAdhakas according to their view. Or worse,worldly siddhi mainly only more often.
Another thing is that in Hindu views,the deity with the same iconography is interpreted differently. An example is here where the image which is considered that of Arya achala is interpreted as that of krodharAja rudra trampling some demonic yakSha. Sometimes the descriptions in the Astika version are superficially different(sarvasiddhi and sarvabuddhi rather than sarvabuddhaDAkinI).
So the transmissions which have come back to Hindus with that Buddhist dress have in key,a different understanding than the Buddhists of what they are worshipping+when they got back the tranmissions from those who achieved only siddhi,their correct understanding put them on higher states of AdhyAtma than the Bauddhas they got it from.(albeit coloured by the ‘shunya’ type of language they inherited)[because they could understand the tattvas better and the devI granted them a place in those higher tattvas]. Another core difference is a practical respect in terms of ritual to brahmins(in the bhojana anga of the Hindu method of attaining siddhi in mantras).
Addendum by @TheRajarshi:
1. The necromancy practices are still prevalent even among H Kapalika-s (few are there still) but neither done in public nor publicized.
2. The actual consumption is mostly a tokenism in ritual setting to gain powers. 3. The mention of “brahmin” is because brahmins were the ones who had the tapa-shakti, other Vs did not. Same way even today in H viracara when taking a kapalapatra one cannot take random one…. else there is no Shakti in it, and becomes useless. Specific type of kapalas are useful. Guru decides basd on nature of sadhaka/na. Same logic baudhas were using. A kapala of a an individual who was a good yogi/sadhaka is highly prized. Or a chandala for certain rituals.
3.The icons of baudha deities trampling on H (IMO) was result of assimilation + abhicara practices which each group was doing on the other. But by time of Kalacakra tantra the Baudhas had come full circle and accepted H deities without any modification/distortion – post-Islam.
4. Coming to practical upasana Vajrayanists also produced terrific siddhas (not lower level at all if one knows how to judge the level of a siddhi) as did Hindus.