The biggest problem opponents are using to block or roll back RCV is transparency and time. Hand counts take longer and may get vastly different results if there are discrepancies. But those concerns are mostly smokescreen from groups that benefit from the status quo. Any hand recount takes time, and if you fully tabulate the entire vote, it's easy to locate potential problems with the computer count.

My concerns are transparency and honesty, and both stem from the fact that only your first remaining choice counts in each round, and one candidate is eliminated in each round. Because only your first preference counts, the most important selection is your first choice. Everyone's second choice gets no votes in the first round and will be eliminated, even if they get 100% of the second choice selections.

Several candidates from the same ideological neighborhood split and dilute the vote from those voters for the first round. If everyone doesn't rally around one specific candidate, all of those candidates could be eliminated in instant runoffs as the lowest vote getter. You have to vote strategically to make sure that the spoiler candidate on your side is eliminated before the spoiler candidate on their side.

Like, let's say we have five fictional candidates, and arbitrarily assign them Green, Blue, Purple, Red, and Nazi. Blue and Red are the front runners, Green is the spoiler for Blue and Nazi is the spoiler for Red. Purple is a third centrist party

Blue voters assume Green voters will pick Blue or Purple as their second choice, and Red voters assume Nazi voters will pick Red or Purple as their second choice. It's in both Blue and Red's interest to see Nazi and Green beat Purple in the first round and then have their opponent's spoiler beat their spoiler in the second round. This creates a scenario where strong Blue supporters are strategically voting for Nazi as their first choice, even though that would be there last preference.

So let's say the preferences roughly break down into 6 categories

30 BPG 30 RPN 15 GPB 15 NPR 5 PGB 5 PNR

With a FPTP election, Blue and Red would convince everyone that Green, Nazi, and Purple have no chance of winning, and therefore voters should pick a frontrunner. And they'd be right, because FPTP sucks balls. But the winner would be whichever frontrunner can convince enough voters to pick their third choice.

With RCV, it is better but still not great. This scenario would be deadlocked at the second round, so Red attempts to convince a few Nazis that their candide cannot win and switch their vote from NPR to RNP. Blue tries a different strategy, and takes some of their own voters to switch from BPG to NBP. Both frontrunner candidates are still vying to convince some of the Purple supporters to change their minds. Anyone that picks some combination of GNP risks having their ballot expire, so they have to pick R or B even if they hate both equally.

So there's still almost no chance that a third party will win, only now it's more complicated. Plus if there's a hand recount, a few votes one way or the other can dramatically change the final tally by changing who comes in last. A better name for RCV is Last Past the Post. It's better, but it's still not representing the true will of the voters, and it's not encouraging campaigns to win hearts and minds. It promotes gamesmanship and back-room deals over voter outreach and turnout.

Approval voting is pretty good, someone else mentioned that one. The only problem I have with that is that it encourages negative campaigning. Every campaign would be attacking Purple, and promoting party purity and loyalty as an ideology. Compromise becomes the enemy, because you have to control the ball.

Star Voting is fair. Every vote counts, and every vote is an accurate representation of the voter's preference. There's only one instant runoff, so a recount might change who is included, but there's no reason to be strategic with your votes. Negative campaigning is discouraged, and candidates are rewarded for finding common ground because ratings are not mutually exclusive. And the best advantage, there's no way for the frontrunners to use demagogeury or political maneuvering to box out new candidates with their clout.

My biggest concern with RCV is that its flaws are dampening enthusiasm for change. People recognize that the current system sucks balls, but if RCV ends up disappointing those who were on the fence about change, they aren't going to look for new solutions. They are going to retreat to the devil they know.