The trouble is that I don't balance-check every option in every 1pp/3pp book that myself or my players pick up. When making characters my players do check with me if there's a new book/class/feat/etc that they're looking at playing, but even if something raises an eyebrow I typically don't want to just shoot it down, because I figure "this designer knows what they're doing, maybe it just looks a lot more broken on paper than it'll be in play." Nope, turns out it's way overpowered, and now I have to either do the design work myself to nerf the problematic features, or the player has to change their character's subclass/archetype/heritage/etc, which can be jarring.
So the more content that you have in the character options pool, the more likely it is that there'll be inevitable power creep (or leap), or some mechanics get extra wonky interacting with one-another, and that's just more work to make it right. You don't want to disappoint players either, but if their character is so powerful that the rest of the party gets toasted just so I can try to challenge the OP PC, that's no fun for the rest of the aforementioned roasty-toasty party