Happens independently of setting.
True, but one can minimize the excuses that the setting in particular gives for it. To acknowledge you can't eliminate a problem is not the same as to say you can't meaningfully reduce it.
That's a game mechanics thing. 'Wait but how can X thing not fail against steampunktech?' is the role of the settings writer.
Exactly. And the more heterogeneous elements that are tossed into a setting without thinking through the logical implications of how they would interact (which is the definition I'm using for "kitchen sink"), the more such mechanics questions are likely to arise.
Speaking as the one mentioned who had issues, yes, such things can get bad enough that you literally cannot envision the setting. When you have late 19th Century steam ships (screw propulsion, turret-based weaponry) and 15th Century caravels (and frigates said to be the greatest of all warships of the time) and melee weapons and bows supposed to compete with late 19th Century revolvers and repeating rifles… yeah, for me, the setting just completely broke down as something I could coherently envision.
We finally found something workable by pegging baseline technology at early 18th Century; squarely in the Age of Sail, but with the earliest steamboats starting to appear. Similarly, firearms would predominantly be flintlocks with variations like the pepperbox as attempts at multi-shot weaponry… keeping sword and bow still competitive with the firearms of the day (particularly the bow’s rapid fire capability in an era of declining armor).
Nailing down a time period (with outliers; throwbacks in primitive locales, somewhat anachronistic future elements available to artificers and the like) did wonders for me being able to try and grokk how elements interacted; 1800 Techno-magic England vs. 1700 Druid-magic France is something that didn’t break my brain in the way that 1890’s England vs. 1450’s France does.
That said, I could see more radical differences working if your setting had something like “reality zones” where certain things didn’t work and others did… if passing into the 1300 France zone caused most gunpowder firearms to fizzle and steam engines to fizzle out then putting the two into the same setting could work.
Similarly, a very high magic 1300 France where magic largely subbed for technology (but was incompatible with technology) vs. a more technologically advanced area would be something I could grokk; you have two mutually exclusive tech bases each with their pros and cons and different societies have different preferences between them. The French may not have steam engines, but their harnessed water elementals allow their ships to match speeds and their ironwood enchanted hulls are as good as the steel plate of the steam ships and they have enough trained war wizards to substitute for cannons.
Which basically goes back to your comment that consideration of how the interaction of the various parts added to a kitchen sink setting are important to how well it hangs together.
As an example from own setting, my decision to allow various flying creatures as PCs led in turn to both other means of flight (various flying beasts, airships and magic), but also into monster design (ranged attacks are extremely common because flyers are common) and PC starting gear (most of the default starting equipment packages include a ranged weapon). Islands and castles that float in the sky are similarly things which exist.
Flight allows bypassing certain threats (ex. hazardous terrain en route to a ruin), while increasing others (everyone in the ruin can see you coming from miles away and probably has ranged attacks, if not flying assets of its own they can use to intercept you).
But, I also mitigate some of flight’s advantages mechanically… requiring greater concentration for flight makes certain tasks while flying difficult without burning resources (flight in combat being more useful for repositioning than remaining out of reach at lower levels, particularly when ranged attacks and indoor encounters make outright evasion of attacks difficult or even impossible).
Sure, if you want to hit PC’s with various overland encounters such access to flight might be a complication; on the other hand flight is usually tiring so you have to land periodically anyway (if you want them to find something in the ground), flying threats and ranged attackers exist (so combat encounters aren’t rules out) and the different vantage point can let you reveal sidequests even more easily (in a way that allows heroic PCs to feel more proactive… you don’t just stumble onto a burning homestead in the wild; you see the smoke rising in the distance and what PC isn’t going to look into that?
The point is, flight could have been something very unbalanced and disruptive, but by considering the ways it could affect things and leaning into them it added all sort of interesting things to the setting (because if you want your setting to allow PC dragons, saying “oh, but they can’t actually fly until they’re level 10” actually feels more lame to me than just not having dragon PCs at all).