People who accuse PoE of "balance" and that that "balance" is at the heart of the problem with the system don't know why balance exists in general methinks. It's not so much for the player, but for the encounter design. If you have 1 or 2 very overpowered builds, how do you balance the mobs? Do you balance them for the other builds or do you balance them for the overpowered builds because it's assumed you'll use them anyway? The point is consistent challenge, not stopping your fun. Where is the fun in steamrolling all opposition? Then you complain it's too easy.
I don't think the balance complaints aim at this particular thing. It's cool when every class is similarly useful. It's not cool if every class plays similarly. Why even have classes when fundamentally they're not very different from each other? Heck, most classless systems (Arcanum, Underrail) offer more gameplay variety with different builds than PoE does. If you wanna do damage you have to raise STR even when you're a wizard whose damage comes from spells... uuuh, okay? BALANCE, I guess.
Also, PoE's approach to balance is just boring. D&D is balanced, too. Yes, fighters are more powerful in the early game and wizards are more powerful in the late game, but 3E added feats which gave fighters some special abilities like cleave, too, making them a lot more useful in later levels. D&D had lots of iterations, and balance changes were made throughout the decades to keep every class fun. PoE forgets the fun part in its obsession to make everything balanced.
At release (apparently this has changed now with patches and the expansions, but I haven't played the game after 1.0 so I can't comment on that - and it's this first impression that put many people off) magic weapons had boring, trivial enchantments like +5% to damage and other boring shit like that. Meanwhile Baldur's Gate 2 had the Flail of Ages, the Vorpal Sword, there was even a fucking sword that talked to you.
A recent game where I enjoyed the combat is Divinity Original Sin. Even the sequel, which had some truly retarded design decisions which I didn't like, was fun to play because combat gave you lots of different abilities that were actually powerful and interesting. Yes, often they are OP - you can destroy an enemy's physical armor and keep him stunlocked with knockout arrows forever, for example. But the enemies also use the same abilities on you so it's actually fun to go against equally leveled enemy groups. Abilities are impactful and battle-changing.
Meanwhile PoE just has... "balanced" abilities. Everything feels very minor, there are no big fuck you spells, there's no resistance spells to cast upon your mage which the enemy mage then strips away layer by layer, no cool mage duels with a fucking demilich who throws insane spells at you while you try to counter them with your own. PoE, despite being magical high fantasy, felt more mundane to me than actual mundane no-magic settings such as Fallout.
It just didn't have anything that felt truly impactful. And impactful, fun stuff > balance.
2) For me it's not only the strategic element that is lost. It's immersion, suspension of disblief. I am a bloody wizard, but I can only cast that spell only when "inCombat = true". Do I have to be reminded every single time I open my spellbook that someone put that artificial restriction there in order to balance the game?
Immersion is misunderstood and frequently overrated. Sometimes it's beneficial to sacrifice ludonarrative coherence in favor of gameplay and there's nothing wrong with that if it's justified, which it is in the case of PoE's system. You can always think up a reason why mages can't cast outside of combat, you can only ever strengthen with magic when your adrenaline is high for example.
If you have to sacrifice ludonarrative coherence, you're retarded and suck at game design. The one who has to think up a reason for why magic is only available in combat isn't the player, it's the dev. The dev has to have an in-world explanation for why his magic system works that way. Just making it so for the sake of Holy Balance (TM) and then not bothering to explain it within the setting is lazy and incompetent.