>This is stronger than plurality or ranked?
Much stronger and more damaging. In a non-score system, a candidate does not care if their supporter ranks the other side as a 1, 3, 5 or 9. They voted for the candidate and that's what's important. In score, it's critical for every candidate to demonize the other side completely to ensure they get a 0 from their supporters (yes, that happens anyway, but now there's a much greater incentive). Way more polarizing.
[EDIT: Note that ads against your in-party opponent can only be so negative, because you'll need the opponent's supporters to vote for you in the general. This is true in general for wavering voters - they dislike too negative ads against people they also like, and that provides a limit of sorts. In score systems you have an incentive to demonize your opponents, even when talking to people who already support you]
>It seems your last two comments are acting under the assumption that IRV passes this criteria.
Actually, I prefer AV, but my considerations are... atypical. It's just I am very much against score/range methods, unless they are a thin disguise for a ranked system*, in which case it's slightly less bad.
* e.g. STAR, highlighted in a different comment below, is a ranked system in disguise. It's clever but way too complicated for voters.