My model isn't that a build ruins the game, my model is that a build compared to other party members ruins the experience.
A level 11 BM PAM GWM fighter can more than double the damage output of a naively built level 11 Ranger. They can hit 5 attacks/round reliably, connect for 20+ damage per hit, and connect reliably.
The naively built level 11 Ranger gets no noticable combat features from their subclass, mixes melee and ranged attacks (has TWF style), and does 2 attacks for about 10 damage at about the same hitrate at the BM's attacks.
If the player playing the level 11 ranger has in their head "I am a competent combatant" as part of their character image, this is completely broken. They are basically a non-combatant next to the more optimized PC.
A party where everyone is in the same league as that Battlemaster is a playable game of D&D.
A party where everyone is in the same league as the Ranger is a playable game of D&D.
A party where both are in it, well, it sort of breaks down. Making combat hard enough that the BM is challenged makes the Ranger into a speedbump. Making combat where the Ranger can contribute is trivial.
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If your position, as a DM, is "I don't want to do anything" to deal with party power differences, then this is frustrating.
On the other hand, this specific case is fixable as a DM.
You give out the Glaive of Lost Souls, a +2 glaive that lets you cast "Soul Cage" 1/rest when you kill someone with it (only 1 at a time), and grants temporary HP on a hit. This doesn't boost the BM's offensive capabilities much more than a +2 weapon, but is awesome cool. If this is the only +2 polearm the BM is likely to find, they should still find it combat-optimal.
For the ranger, you give out Flamedancers. These rings can manifest either a pair of scimitars or a longbow. As a pair of scimitars, they are +2 scimitars that deal an extra 2d6 fire damage, and when using the two-weapon fighting bonus action you can attack twice instead of once. As a bow, it is a +2 bow that summons +2 arrows with Flame Arrow cast on them, and when you cast a spell as an action or bonus action you can also fire the bow once. In either mode, when you are hit by an attack as a reaction you can attack back; if your reaction attack hits, the triggering attack has to reroll with disadvantage.
The ranger goes from dealing 20-30 damage if everything hits (+7-10 if using HM) to dealing 66-85 (ranged-melee) (+10-14 if using HM).
The BM goes from dealing 100 damage if everything hits to dealing 105 damage.
Instead of being in different leagues, they are now in the same league.
If the BM is a sharpshooting XBE, then you'd have to do a different approach. A cloak that lets you cast HM when you hit a creature without expending concentration, and makes it deal 3d6 damage? Why not!
You, as the DM, have that lever.
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Now, quite rightly this will trivialize the combats the party was facing prior to that kind of upgrade. But they where already trivial due to combat optimization of the 2 of the PCs.
Now, however, you are free to 2x, 3x or even 4x the HP of enemy monsters, or grant them resistance to all BPS damage, or whatever. (Don't do the same thing every time).
Come up with story reasons why these evil humanoids all have resistance to BPS. Maybe they are all frothing barbarians, or are life-linked to the liches soul cages with a tattoo.
And the game continues on.
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Things go poorly if you mix the crazy items (or boons) with the high charop PCs. And the temptation to go overboard, and boost the non-charop combat abilities way over the charop PCs, is there (I'd resist it; the goal is not to punish, but to make the game fun).
A side benefit of making items customized to boost the non-combat abilities of the combat-charop PCs (like the soul cage thing above) is that people, when given toys, want to use those toys.
When they kill a creature, they get to ask the killed creature a question and get an honest answer. They get to compel the DM to give them a strait answer for once. For many people trapped by the "combat mechanics is the only way I can reliably get to impose my vision on the narrative" damaged players, that is like heroin, and can get them interested in the story.
Some DM using one of those annoying Cagey NPCs dropping hints? Just kill them and interrogate their soul.
But, get in a fight with some assassins hunting you down? Kill the boss, and get an honest answer who hired them and why.
This might seem like a dead-weight loss. But by giving the PCs mechanical ways to interact with the plot, odds are they start giving a naughty word about the plot.
