About the Carbuncles ...

[Bishop81]

I'd say the loss of Contagion will hurt you more than you think.

Carbuncle has Contagion; The ability is called Shining Emerald

This is why players complain that Garuda-Egi and Titan-Egi are really just a reskin of the Carbuncles.

Their abilities are exactly the same /w the Egis have slightly more potency to their attacks. That's it.

This is also why I feel adding them back would not be a big problem. If you upgrade the potency of the Carbuncles' abilities ... you effectively can use them as you would the Egis.

It's also why I have the suspicion that SE might have something else planned for them, hence don't want SMNs summoning them, it's such a trivial change to implement.