RPGs as a genre are inherently imbalanced and trying to change that is just going to make it lame and sap its spirit.
In original D&D, were the classes really balanced? LOL no. Fighters are the supermen at low levels, while wizards are weaklings who die to one unlucky crit and their spell selection is laughably small. But once you reach mid and high levels, the fortunes reverse and wizards are OP while fighters are reduced to mere meatshields. And rogues? LOL. You gotta take one along to pick locks and disarm traps, but in combat they were total shit. But it's still nice to have a player in the group who goes for that class due to the non-combat stuff.
It wasn't balanced at all. Yet every class is fun to play. Why? Because every class is useful in its own way and has a specific role to play. Therefore it's called a role playing game.
Good CRPGs are similar. There are different classes who are good at different things. They're not supposed to be equally good at the same thing. You can be good at fighting or at sneaking or at spellcasting or at diplomacy. In class-based systems your character's specialties are determined by class, in classless systems you're more flexible and can make jack of all trades who are masters of none. Different characters gain different benefits from different skills. It makes sense that a wizard should have high INT and WIS and a fighter high STR and CON. You can also play deliberately gimped characters like a low INT wizard who can barely cast any spells and is a bumbling fool - bad for dungeon crawl campaigns, but fun to play in a more lighthearted campaign.
Overbalancing leads to everything feeling samey, and yes most of the Codex thinks of PoE as the prime example, where for some reason wizards need big muscles to give power to their spells (??). But it's not just PoE, there's a general "overbalancing" mindset among some designers that leads to samey, uncreative and boring characters. Usually, such design aims at making all the classes equally good at all the tasks. Encountered a lock? Wizard casts unlock spell, fighter uses bash, thief uses lockpicks. All these abilities are gained at the same level of the respective class: thieves learn how to lockpick at level 2, unlock spell is a level 2 spell, lock bashing is a level 2 warrior ability. All the same. No reason to pick one over the other. Really lame.
You want different characters to play differently, you want different levels to play differently, and you want a large amount of options for the player.
Arcanum is known to be a hideously imbalanced game yet I fucking love it and playing different characters is such a joy. It's not particularly difficult either so playing deliberately gimped characters is viable and fun, too. Dwarven wizard even tho dwarves have an inherent pro-tech anti-magic aptitude? Yep. Elven techie even though elves have a pro-magic anti-tech aptitute? Yeah. Going for all-powerful world-shattering wizard who abuses the overpowered level 1 harm spell to murderize everything? Fun. Playing a techie who has to scrounge for scrap in trash bins and is pretty underpowered until you finally get those OP crafting schematics at the end? Fun, too. Archer whose rate of firing arrows is so rapid, it feels like a machinegun bow? Hilarious. Trying to use a gun when you have a magical aptitude so every time you pull the trigger there's a chance it blows up in your face? LMAO. Diplomat who just skips the worst dungeon in the game by using in-depth dwarven philosophy? Cool.
Arcanum is hilariously unbalanced, yet playing different types of characters is so fun because they play so differently. Some are inherently OP while others really struggle until the endgame, but that just contributes to the fun factor even further. It's cool when your choice of class can affect the difficulty, you can go for challenge runs where you attempt to win with the most gimped character possible.
An overbalanced game offers little replay value because ultimately, all characters feel the same.
Oh yeah, and nothing says that every character has to be able to see all the content. Arcanum has several dungeons you will only enter if you become a master in a certain skill, as they're connected to the skill master quest. Content exclusive to some classes or characters with certain abilities is always cool. Can't do the burglary quests unless you got thief skills? Yeah makes sense, and gives you an incentive to play as a rogue (who in most systems is a bit underpowered in combat compared to other classes). The "everything ends in peaceful negotiations" ending is only available to characters with a high speech skill, and other chars have to make do with one of the other endings? Totally makes sense.
It's completely fine to have side quests or entire questlines that are locked for most characters, except those with the appropriate skills. Making sure every quest can be solved by every character is a form of overbalancing too, and makes you feel like your skill choices don't matter that much. We all know the examples. The enemy manor where you can either sneak inside, convince the guard to let you in, use a spell to get in, or fight your way in. It's cool when a quest gives you so many alternative options. But when EVERY quest gives a unique solution for EVERY character type, it starts feeling artificial. Like... why does EVERY single enemy base have a backdoor you can sneak into? To make sure players who play a rogue can finish that entire questline? But now it just feels lame and boring and formulaic rather than "hey look at the amount of choices I have", because every quest was designed with a checklist to make sure every class has a way of solving it. Lame.