I think the thing that kind of bothers me about Operation Mi'hen is a lot of the hindsight issues of the game itself. Sin, is not a difficult boss mechanically. Now, that's true of a lot of FF story bosses, they are not terribly tough by design some of the time and this instance is no different. I think my issue is that narratively, Sin has a weaksause weakness, of which if the Crusaders and Al Bhed had simply known at the time, this operation may have partially succeeded. And that knowledge sort of diminishes the fact this whole sequence is designed around showing off how overwhelmingly powerful Sin is and driving home the false idea that the Final Summon is the only means of beating Sin to the player.
Bit surprised to read this considering that I feel FFX goes out of its way in basically every way to sell it and does a good job of it. You only fight Sin itself while it's being massively nerfed with an extremely specific weakness to the hymn, and like 30 minutes after you beat it down it has already regenerated (and forming wings this time, showing it can reshape and adapt to its threats) so like no even if the Al Bhed HAD known about the weak spots on its fins and managed to tear it off with their cannon the same way we do later with the airship, they would have ultimately achieved nothing, even less than a summoner sacrificing their life. Couple that with the fact that Sin is the only enemy in the game (series?) whose overdrive is completely impossible to survive by any means whatsoever and is pretty close to causing severe world devastation *while* this fight is ongoing and it comes down to that you're basically just trying to get in as fast as possible while it's still nerfed. To me it really shows just how far beyond anything else this being is and it's only an extremely specific set of circumstances that allow us to even fight it, obviously those circumstances have to exist so we can have a game/story at all but yeah. But I guess it's all down to perception of how well the game sells these things through game design; curious if you still feel that way once you get to that point of the story this time.

The whole party members having to participate to gain AP is a bit rough, as while I enjoy swapping characters around dynamically a lot myself and find fun uses for everybody, I still often find myself in a situation where only 5-6 characters would get AP unless I force bring in a final character and just defend with them and maybe take another enemy attack in the process. Yuna in particular tends to be the offender in this since I don't enjoy using aeons and out of combat healing is entirely punishment-free in this game so it often feels a bit much to do it in combat just to have something to do. I imagine it was done to encourage swapping around a lot but the game already does encourage that with its enemy design in general so idk. Maybe it was also just still in a time where the idea that only participating party members get EXP was so much the norm that the concept that you can just give everyone all the EXP all the time just wasn't on their minds, it's only FF13 where they finally did that in the end. Certainly something they could have fixed up with the HD version though.