1) It's gotta be a mainhand weapon.
While this is true, it's also true for all of the other RMEA so not really that significant of a difference.
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2) The Enparalyze thing could be annoying on targets that you have to avoid debuffing at all costs (but haven't seen anything like that since like Delve1?)
Also true, but like you said it's very rarely relevant (e.g. Tax'et) - could just use a different mainhand in those awkward situations, like we would do for our Mandau THF back in Delve-heavy days.
Most of the time, the occasional paralyze proc is a minor plus. Nothing to really make a big deal about and the duration is usually pretty short, but generally not a bad thing.
* CAN be overwritten by Para 2 from a RDM, so that's not a big downside
* IIRC, at 75cap Kikoku's en-paralyze can't be overwritten with Paralyze I spell, so that
could be a downside on particular mobs that must be paralyzed for some reason (e.g. locking a TP move). Caveats: I can't think of any such mobs off the top of my head (and the random para from Kikoku might actually help there if you don't have a mage that can stick the spell), and I might be wrong and the en-paralyze effect might have been changed at some point.
Similarly, Subtle Blow +10 from relic AM is absolutely a very minor bonus, but it's at least something positive (assuming you're not already at SB+50 cap from gear/Auspice). Laughable when it was the only Relic AM effect, but now it's a "huh, that's nice I guess" feature.
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3) It lacks +accuracy bonus (altough the 119v3 has 269 skill so that's plenty of acc to begin with)
Again, also the case for Kannagi, Nagi, and Heishi, so while true it's not especially relevant since it's the same for all RMEA.
The extra 27 skill on all four is a decent chunk of accuracy AND attack though, and for situations requiring high accuracy you do have Shigi as an offhand that will give a massive accuracy +77 boost to both hands. That huge acc and low delay is why Shigi is often best offhand for any of the RMEA (and if less acc is required, Kanaria becomes more viable).
Kannagi > Kikoku/Heishi > Nagi. All assume AM3 up.
Depends on what ws you are using as well.
Relic III metsu spam is very potent, and only falls slightly behind Heishi/Ten and slightly above Heishi/Shun spam. The gap between Kannagi AM3 up and kikoku/heishi isn't very big either. SC potential with Metsu being used is great as well.
Yeah, Metsu does give some extra SC flexibility and is a solid WS.
Kannagi wins in most but not all situations, though it's not a huge gap between any of Kannagi-Kikoku-Heishi. Playing with buffs/gear/targets, I do get Kikoku as a clear winner sometimes. Heishi and Kikoku are pretty close, and again one or the other can win - though generally my spreadsheet testing has Kikoku a little above Heishi.
Nagi often comes in 4th now EVEN WITH AM3 up. And maintaining AM3 can be a real hassle, especially in much of today's content. If you can't keep AM3 reliably, non-RMEA choices beat it too.
I'd put it as:
Kannagi > Kikoku > Heishi > Nagi. With the gap between the first three relatively close, and Nagi having a significant drop-off.
If gil is an issue, Kannagi is a bit more expensive than Kikoku and certainly isn't game-changing enough to be the make or break issue of whether or not you can use a NIN DD (i.e. neither Kannagi or Kikoku is gonna make you beat good BLUs for DD, and either one will be competent as a slightly below the top of the pack DD).
Nagi is both most expensive to get to 119(III) and lowest performance, so is really only for a hardcore status symbol or if you happened to already have a completed 75+ Nagi and want to finish it off (it's still GOOD and generally better than non-RMEA). Though the 10,000 beits alone stings, even if you were sitting on a non-Afterglow 119 Nagi already.
Based on the groups you have access to and the ability to kill the hardest Escha NMs, Heishi could either be the easiest/cheapest to obtain RMEA, or completely outside of the realm of possibility (whereas with a lot of time/money/effort, you can solo the other three). It's not generally gonna be top of the heap, but it's solidly top 3 and one of the better Aeonics, so if you have an Aeonic-clearing group and don't want to do a Kannagi/Kikoku, it's not a bad choice at all.
NIN atm is an excellent DPS with some unique feats (magic bursts, invincible against one-hit moves that would kill others). Some could say NIN with Innin up is likely the third DD in the tier list (after BLU and DNC).
It can also tank pretty well against some targets but alas they're tipically not particularly relevant ones.
tl;dr
A nice melee DPS in a meta-game where melees are mostly irrelevant
This is more or less a good description, though a couple clarifications:
Melee isn't necessarily always irrelevant, and seems to be making at least somewhat of a comeback. At least in month 1 of Ambuscade melee DD has been VERY useful and SC/MB deemphasized a bit, and perhaps that trend continues (I'd guess it will). It's useful for some more melee-friendly Escha NMs and is fine as a CP party DD/SC starter.
I agree that it will be very rare for a well geared NIN, even riding full-time Innin, to out-DD a BLU or DNC with similar competence/gear - unless the need for Utsusemi or Migawari come into play. There are other DD that are similar to NIN fighting for that next spot too, like WAR (better lately than you might give it credit for), THF, etc.
That being said, NIN can fill the DD role fine for most stuff if you're not insisting on "*optimal* melees only!". My friends have no issue with my NIN being a DD in Ambuscade, CP, some Escha stuff, etc.