I don't necessarily mind the illithids, but I don't really think they suit the Forgotten Realms the way they suit Greyhawk/Oerth. Not enough gross fungal deities in the former.

(I think Zuggutmoy supposedly does exist in FR, but I've never seen her get much focus. Ed Greenwood seems far more into his gorgeous magical goddesses and demigoddesses than in visceral pseudo-Lovecraftian horror.
And there's nothing wrong with that! But it does make the illithids fit oddly.)