I would love to write a detailed historical/theological rebuttal to the rehabilitation of Jansenism, but I'm also writing a dissertation on Arianism, so it's hard to find time. For the record, though, the main historical/theological points are & have been fairly straightforward:

(1): The condemnations in Unigenitus, read together in context, are not at all hard to follow, nor are they ambiguous. They are clear condemnations of a view that denies the distinction of grace & nature taught both by Thomas & (more importantly) Trent in its anathemas.

(2): This has numerous consequences, like the denial of the possibility of good works apart from grace, that the Fall was a corruption & vitiation of nature rather than a loss of original grace, the view that grace is therefore irresistible in relation to corrupted nature, etc

(3): Anyone who has even a passing familiarity with Luther or Calvin, or Calvinism as a religious system, can recognize the commonalities. The key distinctive of Calvin's theology, in fact, is the denial of any meaningful distinction between nature & grace, or the orders thereof.

(4): In practice, this led to religious & Sacramental practices that, like Calvinism, placed a huge premium on constant, stringent self-examination about the presence or absence of (perfect, binary) grace. The worst of these abuses was the frequent denial of absolution.

(5): Jansenism was very tied to a French Church that had long been legally & strictly Gallican. Similar doctrines were used in Italy (a la Pistoia) for efforts at "Church reform" by the secular power; key to said reform was the severe legal curtailment of Papal authority.

(6): The term "Jansenism" has been, & continues to be, used primarily of the condemned doctrines & practices. It is very possible to find good aspects & in all heretical groups & times & places, but ideally that would not extend to the defense of the heresy itself.