I don't like instant runoff. It seems like it systematically rejects compromise, and I think finding the proper compromise is an important goal of a voting system.

For instance, imagine a sectarian society split roughly equally between 3 religions, with 4 candidates running. The first three push theocracy favoring their respective religion, while the fourth favors tolerance and secular government.

If, statistically, most of the citizens favor their flavor of theocracy, are happy to put up with secular tolerance, and are vehemently (maybe violently) opposed to living under another religions theocracy, it seems that clearly the best choice is that fourth candidate.

If everyone votes their honest preferences, the first thing IRV does is throw away that fourth candidate.

The vote might not go that way if enough people recognize the situation and vote strategically, ranking what they see as the proper compromise artificially high. But that's true of any voting system and undermines major selling points of RCV.