Carbuncle is weak because he's your "noobSMN" avatar.

But Cait Sith being weak........
Um...
I got nothing.

Hopefully, with a bit of tweaking and Her last 2-3 BPs...
She can become what she was always supposed to be: the equivalent of Diabolos and Fenrir.
I think this can easily be accomplished (and thusly we can all calm our collective tits over Her current state) by Her "merit"
BP.
All Avatars have a "70" and a "Merit" BP and those are pretty much the only offensive BPs a SMN will use.
The 70 is usually a physical move (predator claws, rush, eclipse bite etc) but Carbuncle and Diabolos's are magic (meteorite and Netherblast).
And the Merit is... Well... The merit magical BP. For Carbuncle, Fenrir and Diabolos, they don't have real merit BPs, but their lv76ish (it varies) BPs function in a similar capacity and have similar damage output.

Cait Sith doesn't have Her post-75 BPs.
And this is Her salvation.
How often do you use any of the non70 or nonMerit BPs at 99?
Never. (Obviously there's some exception, but an exception doesn't invalidate my point.)
So we don't even need to make lv?? Holy better... Because once She has a "merit" BP, we will never use lv?? Holy ever again.

Personally, she has 7 BPs right now, I would give her 3 more.
1 ward: sneak+invisga
Why? Well, She was constantly going invis during WotG... Plus, it fits with her supersolostyle RR BP. Though this can be axed if Atomos will be doing that.
2 rages: Holyga/ja (the "Merit") and Banish V/Holy III
I'd like the "merit" to follow suit with Carbuncle's model of being just a bigger version of the "70"... So a straight damage Holyga would be like lv?? Holy... Just without the random. (Obviously that means no stupid damage from it... But I'm good with that trade.) and the Banish V/Holy III just to give her a powerful single target nuke.

If they add BPs like this, I think she'd be the equal to Diabolos and Fenrir at 99.