Exactly, Sazh is far from bad during the main game, but the big fights that are much of an issue during the main game Haste isn't really an advantage. You go into those fights prebuffed with both offensive and defensive shrouds. So you are more looking at a good healer/Ravager with options of good defensive buffs (so if it is a long fight you are back to being vulnerable after shrouds are gone) compared to Sazh who brings Com/Rav to the table which you have preeeetty solidly covered with Lightning with the ability to rebuff your key offensive buffs after your shrouds run out. Suffice to say if a fight took the 5 minutes or so it takes Shrouds to run out you are looking at a boss that you are turtling against or something where blitzing is just straight up not your best option (That or you are ... really bad at blitzing the enemy?) and if you are looking at one of the enemies with Dispel I would be questioning the net benefit of extra turns spent rebuffing Haste with ones gained over just continuing to deal without the Haste.
He really shines in Randoms where you are likely to get good use out of Haste, he definitely isn't Snow who you either have to leverage into the party or just pretty much never use. Compared to the raw effectiveness of Lightning's setup for most of the game or the things that Vanille, Hope and/or Fang bring to the table from their toolboxes there is no real shock that people are going to be a bit underwhelmed by "Brings Haste and redundant party building skillset".
He is a functional choice and there is definitely situations that he is optimal to apply to, but they aren't situations that you honestly can't get by without him. The person's slot he best fills is Lightning really and using him instead there is akin to using Snow in a party. You have to either forgo a strong defensive option or leverage him into the party because you want to use him. That is absolutely fine, but not everyone is super peachy keen on dropping the raw powerhouse that is Lightning or rejigging their slightly less optimal unhasted party in the situations where you don't need the defensive options that come with having a Sentinel and/or Saboteur or Hope's Synergist skills available (Because you know, reshuffling Paradigm Shifts is not exactly the most fun thing you can be doing.)
Sazh would likely shine far more in a game without Shrouds, but the game has them and they are broken, so boohoo.
Edit - For reference, I used Sazh for a few reasons, one being I like having haste for randoms sometimes and he was the only character in the game I gave 2 shits about, so hey may as well give him a spin right? Well for a game that is all about constantly shifting battle field roles and all characters multi tasking, all but one of Sazh's Paradigms were with him as Ravager with one option of him as Synergist, usually in a Com/Rav/Syn setup that was used to flip between that and Com/Rav/Rav for trash enemy face rolling.
