Thing that annoyed me with FFVI is that while all of the characters have unique skills, most of those skills are either useless outright or BECOME useless. The ones that are useful tend to be really unbalanced too (Edgar's autocrossbow and chainsaw are overpowered. Everything else he has is fairly useless. Same for Gau's Rages and Sabins punchenomics).
I think skills eventually becoming less effective, or even useless, is alright as long as new skills come to replace the old ones.
The problem is that you have guys like Cyan and Strago, the former of which has a skill set that is functionally useless outside of the first two skills because it takes too long to use, and the latter which comes with a second set of magic that, outside of a few novelty spells and better elemental coverage, is more or less redundant compared to the normal set of esper magic (aside for the bargain bin Ultima i.e. Grand Train) nevermind the fact that he first shows up when you have people who are maxing out the available WoB esper magic. Actually the whole Crescent Isle arc seems to have been designed to take place much earlier in the game before you start collecting magicite.
Basically FFVI's personal skill sets can be looked at like this.
Terra: Morph. You don't get it until midway through the game but can be situationally useful when you need things to die now. Too bad it runs off of the game's AP equivalent.
Locke: Steal/Capture. Aside from some novelty items here and there and a plot point or two early on it's functionally useless due to nothing worth stealing most of the time.
Edgar: Tools. Gamebreaking. Auto Crossbow for early trash mobs, Chainsaw for fuck off damage, Drill for when the Chainsaw doesn't work, Debilitatior/Air Anchor for bosses and things you're too lazy to kill yourself.
Sabin: Blitz. Gamebreaking. Pummel for early/mid game bosses, Fire Dance/Air Blade for scrubs, Bum Rush for endgame bosses. Only caveats are that most run off of Mag Pow and Blitz Input can be a little finicky at times.
Celes: Runic. Neat gimmick at times when you're facing a magic heavy boss, but useless later on when you get your own magic or when the solution calls for killing things faster rather than trying to tank a spell.
Shadow: Throw. Pretty useful early on for both damage and utility but both will fall off later in favor of magic. Still Invis Edge is good for most of Shadow's time in the WoB, and the Skeens are good for clearing early game trash when Edgar isn't around.
Cyan: SwdTech. Takes way too long for anything beyond Retort but Dispatch and its defense ignoring property are decent enough. If only this showed up in a later game when everything took significantly longer then the charge times wouldn't have been so crippling.
Gau: Rage. Potentially gamebreaking. You just need to know which Rages are worth it and what to bother grinding for. Too bad the game wont tell you anything and there's hundreds to pick from.
Setzer: Slots/GP Rain. Depends on whether you're using the Echo Screen/Joker Doom trick or not. If so, gamebreaking. If not, you might be able to reliably proc 7 Flash but both are otherwise crap.
Mog: Dance/Jump. Wind Dance is actually fairly useful during the remainder of WoB but otherwise ranges for mediocre to terrible. Especially since it comes with so many caveats and the good dances are too rare. Makes for a good lancer though.
Strago: Lore. Comes too late to be of much use. The fact that Strago himself has such terrible availability makes it even worse. Aqua Rake/Aero good for Water and Wind damage, Blow Fish good for unblockable damage, Grand Train good bargain bin Ultima substitute.
Relm: Sketch/Control. Gamebreaking, but in the literal way. Sketch is buggy trash, Control is only useful for nabbing Lores. At least she has solid Mag Pow and better availability than her grandpa.
Gogo: Mimic. Good for lazy Bum Rushing or double casting Ultima for free. Maybe getting off a second Quadra Slice but why would you bother?
Umaro: Punting your allies into enemies. He's fucking Umaro. Deal with it!