Sigh. Let me get the quotes...
If it Exists in D&D, It exists in Eberron
1. If it exists in D&D, then it has a place in Eberron. A monster or spell or magic item from
the core rulebooks might feature a twist or two to account for Eberron’s tone and attitude,
but otherwise everything in the Player’s Handbook, Dungeon Master’s Guide, and Monster Manual has a place somewhere in Eberron. Also, this is the first D&D setting built entirely from the v.3.5 rules, which enabled us to blend rules and story in brand-new ways.
(Eberron Campaign Setting, pg 8)
1. If it exists in the D&D world, then it has a place in Eberron. Eberron is all about using the core elements of the D&D world in new ways and interesting combinations, with some unique elements thrown in. It's still a D&D setting, so any information for players that appears in another D&D core rulebook or supplement—from the classes and races in a Player's Handbook to the new powers and other features in a book such as Divine Power—should fit right in to your Dungeon Master's EBERRON campaign. (Of course, your DM always has the final word about what parts of the D&D game are allowed and not allowed in
the campaign.)
(Eberron Player’s Guide, pg 4)
7. D&D with a Twist. Every race, monster, spell, and magic item in the Player's Handbook,
Dungeon Master's Guide, and Monster Manual has a place somewhere in Eberron, but it
might not be the place you expect. Eberron has a unique spot in the D&D multiverse, and
many familiar elements of the game play different roles in the world. In particular, mortal
creatures are products of culture and circumstances, rather than the direct influence of the gods. As a result, you can't assume that a gold dragon is good or a beholder is evil; only in the case of celestials, fiends, and certain other creatures whose identity and worldview are shaped by magic (such as the curse of lycanthropy) is alignment a given.
(Eberron: Rising From the Last War, pg 5
Where people get that Eberron isn't a kitchen sink with a twist is beyond me.