Kingmaker was, for me, one of the straightest examples of "more of the sum of its parts". Neither the fights, nor the plot/writing/companions, nor the kindgom management were very good (I played with the turn-based mod to make the fights at least somewhat interesting), but the game had just the right balance and variety between all these things to keep me hooked until the end.

Personally, I would say it succeeds despite its choice to faithfully adapt a tabletop ruleset, and not because of it.

That said, Divinity: Original Sin 1 and 2 are some of my all-time favorite RPGs, gameplay-wise, so I'm interested in how Larian will approach this challenge.