Luna is openly crazy, that's easier to deal with. Gracie though.....that is evil mastermind genius crazy and that is scary as hell
There is a sense of predictability with Luna that when she gets in the "wrath mode" what we can expect her to do to her foes.
With Gracie, there is none of that which we have seen till now, and that unpredictability can come off as a wild card.
We've never seen her go through intense shock or despair (though this might turn out to be untrue if we get to explore what child Gracie went through in flashbacks at the time of Luna's kidnapping by the Mysterious 6), and that I think is scary, yet one of the most compelling things about her character, with the questions that looms, what would happen if Gracie is really pushed over the edge, with someone she cares (for example, MC) for is seriously hurt?
Will she go ballistic and just start killing every foe (or possibly even allies) in sight, or will she go full Jack the Ripper/Hannibal Lecter style stalking her prey (okay, probably not cannibalism lol)?
That, in my theory is what Cordia is trying to protect Gracie from, because with Gracie being in family business, the chance likely increases of her being pushed over the edge, with Gracie's mentality perhaps being more fragile than it appears to be (though I guess we can say it's not going well, with Wilfred continuing to want Gracie to get involved, and Gracie herself sneaking off on missions being influenced by how MC is joining the family business).
Gotta admit, while Luna is always my favorite girl, I'm looking very much forward to how Gracie's arc goes as well.
Some people dislike Gracie because of this scary aspect, but I find her to be a character who is just as good (and written as welll) as Luna, Isabel, Cordia.
It's just the matter of tastes in how favorites get determined, and even though I like Luna & Cordia slightly more than her, that doesn't undermine Gracie's character even in the slightest.