It goes back a bit too the premise of this thread: walled gardens have a hard time including new options because everything already has a place.
For example, the shadar-kai Tome of Foes comes with some very specific lore baked in and some assumptions about the multiverse. Easy to fit in a kitchen sink, but that walled garden has some hard work if the DM wants to make it fit. What is the shadar-kai's connection to other elves? Does the setting have a Shadowfell to come from? What about the connection to the Raven Queen? Any and all of those things need to be considered, modified and or removed. But a generic "shadow elf" that is just an elf who has some shadowy powers and looks like a Hot Topic in the artwork has a lot less baggage to consider and fits in more gardens.
You see this a lot with the notion that classes are too specialized and certain ones (druid, monk, barbarian, etc) should be made generic or rolled into other classes to fit a wider selection of types of gardens. That the Core rules shouldn't assume things like druid armor restrictions or the monks ki. This is strongly evident in the notion that races shouldn't have cultural elements like weapon proficiencies or the like.
All this wraps back up up say that the more bland the base, supposedly is, the more things you can make it go with. I don't share that belief and I fear that people's quest for a core rules that won't interfere with thier garden is going to turn both the garden and the sink into flavorless muck.
Egads that's a bad mixed metaphor!