Nāṣibism (hatred of ʿAlī and, by extension, Ahulbayt) is a broad term that can span a variety of different groups whose hatred of ʿAlī stemmed from different premises.
Accordingly, the scholars exercised nuance as they dealt with these different groups on a case-by-case basis.

>The early Munāfiqīn despised him due to his proximity to the Prophet ﷺ.
>The classical Kharijites despised him due to their flawed understanding of scripture, incorrectly declaring him an apostate.
>The early Syrians despised him for slaughtering their ancestors at Ṣiffīn.

These groups cannot and should not be lumped together, as they don't actually have much in common.
Nāṣibism definitely is a form of heresy and misguidance, but it does not always entail disbelief and apostasy. Its implications will vary depending on its underlying premises.