There are a lot of things in the book titled "Dragons
of Eberron" that are like that where WotC devoted significant pagespace to showing the ways that Eberron's Dragons are just like the dragons of other settings rather than devoting that pagespace to
showing how different they are within the Eberron setting itself. Luckily Rising avoided that sort of thing & there is every reason to expect that upcoming works from Keith like Exploring Eberron & such will also avoid it.
It's not about taste sensibilities so much as identity of those restaurants themselves. I question if you've ever been to Mortons
(a generally upscale steakhouse chain) or Legal Seafood
(a casual to semiupscale seafood chain) if you think that sort of food court ambiance & setup would not massively degrade the identity of what they are.
Eberron has a very different planar cosmology than FR/Plaescape, the gods are not a hands on active provable fact you can metaphorically call up on the phone, monsters are people, and absolute morality is flatly unwelcome in the deeply shades of lore yet the reverse is true of all those in planescape/FR ... Thoughtlessly adding a bunch of lore from one to the other would degrade the recipient setting. Darksun has gods are in hiding or dead, monstrously powerful sorcerer kings that are only held back by the fact that they can't get elsewhere, pretty much assumes everyone is too busy worrying about simply surviving to worry about alignment, &
basically* no planar structure to speak of, Thoughtlessly adding a bunch of planes from eberron or planescape as is to darksun would degrade the setting
As to that article of eliminster/mordenkainen/dalimar meeting.... All of those settings have strong links to absolute morality, active provable gods you can call on the phone, & they treat magic like cargo cultists treat science. Throw in
Ourelonastrix, &
Borys or
Rajaat... the new arrivals look at each other with a polite chuckle chuckle at the primitive understanding of the first three when they say absurd things like having recently proved the arcane equivalent of "π is about 3." An hour into the nonsense of primitives Ourelonastrix shrugs & telepathically offers to help the frustrated SK quietly defiling the potted plants simply to further desirable outcomes in The Prophecy. The SK then proceeds to cast mind rape & worse on the first three to enslave them with the help of Ourelonastrix... Months years or centuries after Toril/Faerun are like wastelands like Athas & completely destroyed by the SK's greed for power Ourelonastrix has never given a second thought to what he considered on par with swatting a troublesome mosquito sees the desirable actions have passed and are now heading towards an undesirable outcome in The Prophecy that can most easily be solved by teaching the SK how to burn the Crystal Spheres containing FR/Greyhawk to a cinder that restores Athas back to the blue age(or doing it themselves). So The Dragons come along to destroy everything in both worlds because now those worlds are doing something to threaten The Prophecy.... With the gods of those settings Dead, Bound, or in hiding for a long time they offer no meaningful hindrance to that cinderification either.
Just because FR & Greyhawk are virtual palette swaps does not change the fact that there are settings simply too incompatible to carelessly dump lore from setting to setting. Being part of the same "Shared" great wheel multiverse isn't the problem, that problem is the careless export of lore from one specific setting into sourcebooks dedicated to some other setting then pretending all of the lore from the setting being shoved aside will not react logically to the new shipment of lore.
@doctorbadwolf Your right about the Umbragen & I don't know much about the Shadow elves of Mystara, they could be as different & unrecognizable compared to the drow of menzoberranzan as the Umbragen are to those drow. Diluting those differences by replacing chunks of lore from one setting with ill fitting chunks of lore from another setting does nothing good for either setting.
* The Grey, The Black, & a hard to reach elemental plane rather than a bunch of discrete planes like eberron & the great wheel is
basically none, not looking for a lore fight ;D