Framing is important, and I think STAR would be most optimal with good ol' Likert scale (five or seven point) rather than 0-N stars. But I think even simpler is the "3-point Likert" behavior of V321: approve, disapprove, and "meh, ok."
I don't like how IRV forces me to rank candidates I may know little to nothing about (e.g. downballot randos not on "my team"). I don't really care if Kick Puppies Party is above or below Trip Grandparents Party.
V321 is certainly more intuitive in the ratings (it's basically an upvote/downvote system) if not the counting, but it's still quite strategic. On the plus side, the most basic strategy of simply upvoting who you want to win and downvoting the candidates you hate ensures the most divisive candidates are removed from the pool and the obscure fringe candidates are unlikely to benefit, but there's a fair bit of juggling how many candidates to upvote or downvote depending on what you think of the candidates seen as most likely to win the final round
That is an implementation issue. IRV can be implemented as either requiring complete ordering or allow partial ordering (you have n candidates, you can assign each one level 1..n, but you may use the same level for more candidates to describe partial ordering).
Technically, that’s a complete ordering with unforced preferences, not a partial ordering.
A ballot with a partial ordering would be won for which for some pair of candidates A, B, none of A> B, A<B, nor A=B would be true. Equal preference and absence of preference are different.
A ballot that worked this way would be a meaningful extension in existing ranked preference methods that aggregate pairwise preferences from individual ballots, lile ranked pairs. (Whether the effect would be desirable is not something I’ve really analyzed, but that kind of method would naturally make distinct use of the information.)
Yes, it is weak ordering, not partial ordering.
> Equal preference and absence of preference are different.
Well, it may be understood as absence of preference, provided the absence of preference is transitive.
A ratings ballot, on the other hand, involves massive amounts of thinking. First of all, the concept of what might represent an "honest" approach to intensity of preferences is open to debate. It's US culture to give their Uber driver max marks and tip the serving staff 20%; is this an honest reflection of the intensity of the average American's enthusiasm for service workers relative to people from non-American cultures or simply what they've been taught to do? And real world electoral situations give you all kinds of incentives to do other things (like give your preferred candidate max marks and all the other candidates 0, or all the candidates you don't hate at least 9 out of 10). Is the person who bullet votes max marks for his preferred candidate because that's what the campaign literature suggested really worth more votes than the person who carefully evaluated all the candidates on their merits but doesn't think any of the candidates are worth more than 6 out of 10? With approval voting it's arguably even worse because there's a massive dilemma over whether to vote positively for the candidate(s) you want or negatively to eliminate the risk of candidates you want to lose. Obviously this applies unevenly to different demographics: put someone downright nasty towards a particular demographic on the ballot paper, and suddenly that demographic is under pressure to Approve every other candidate, whilst others continue to happily only vote for candidates they actually want.
If you design an efficiency metric with the arguably unjustifiable assumption that votes always represent either a perfect expression of the intensity of their preferences or a simple uniform strategic vote, and you assume that the proportion of people voting strategically is unaffected by voting system, it is unsurprising that voting systems whose main shortcomings are strategic complexity perform well. Even looking at that data, IRV actually outperforms nearly all the systems if you assume IRV voters are honest and voters using the other system are very strategic which isn't unreasonable given the incentive structures of the different systems.