My memories of 3.5 was that the player-facing options in the splatbooks really added power when someone was trying to optimize. Especially some prestige classes. This isn't saying all the material was more powerful, but some of it definitely could be either used or cherry-picked.
I voted no. In hindsight, some of the more exploity cleric stuff was in splats (like nightsticks) so I should have voted yes.
That said, I was still trying to vote in generalities rather than say "there's no busted content you should ban." Ban Sarrukh, and don't let people become punpun through polymorph, for instance.
Someone else mentioned that all the most powerful options were in the core books (so that includes the Mystic Theurge and other DMG content, not just the PHB).
This was why I voted no as a generality. The overwhelming majority of the post core books are weaker than Sorcerer and Wizard, nevermind Cleric and Druid.
I really remember the splatbooks adding to power creep, and that someone intentionally optimizing a character with full access to the splatbooks would be able to create someone significantly more powerful than if limited to just the core books.
You could certainly do much better than a fighter or rogue or monk. But they were the worst of the PHB by far.
Infinite PrC stacking could get pretty cheesy, but if you're following the DMG, PrC access will be campaign limited and not freely available, so you probably don't let them take 5 PrCs.
I think 3.5 had a problem of steadily declining quality control as the books came out faster and faster and they used more and more freelancers; and I certainly wouldn't include everything 3.5. Frankly I wouldn't include
most 3.5 content, and would default to 3.0/3.5 FR books (removing problem options) + core + select UA options + psionics + manual of the planes + Arms & Equipment + maybe the 3.0 Class Books like Song & Silence or Tome & Blood. If I were running Dragonlance or Ravenloft or Eberron, the FR books get swapped for their campaign books instead.
There are some other things I would bolt onto that, or swap out, but they're not content published by Wizards of the Coast.
Addendum:
I'm not saying there aren't things I would change. There are many. I am building my own modernised 3.x-Based RPG; and it is substantially less compatible with 3.0 or 3.5 characters than PF1 was. Some targetted spell changes, a few core mechanic changes including making spell prep take 10 minutes per apell level like AD&D, and there being no 'knows whole list' casting (clerics and druids would have divine spellbooks) (if I run 3e again these spellcasting nerfs will be in, and I would nerf wands), different skill list, overhauled magic items and wbl, it's classless and
kind-of levelless... But it's fully compatible for me to grab and use 3.x monsters, and other parts could be imported individually with some effort.
But I wouldn't get too bent out of shape over the /power/ of characters who have access to more than the core books, just selectively remove some problem options, and generally avoid the 3.5 era 'generic' book line, and you're good.