Jae is outstanding as a skill monkey. This is known. But she is very mediocre in combat and her quests range from annoying (stealth quest?) to just forgettable terrible. The line in the DMB part is arguably the only mildy amusing part of her entire quest chain. Her nature makes her highly incompatible with certain playthrough styles, because she is not good for Dogmatic and she is basically bereft of story in heretical route. But more importantly, you don't need a skill monkey to play though this game and even if you do the main character can easily suffice, especially if you go Forgeworld origin.
Ok so lets you say you are rolling Iconoclast and want to get your inner darky on... Unless you are playing with the difficulty dialed way down, who are you going to sub in for her? I don't think there is a less effective NPC in the game other than Solomourne. Maybe you could argue Idria? Even Wulfgar can work as a mobile piece of terrain and an Argentia light, especially with the added gear options the last DLC added. On hard she is basically useless outside of handing 2 AP to another actual useful character each turn for most of the game, which Cassia can already do without sucking. Argentia with a pair of hand flamers makes a better dual pistol character (but why bother when the golden bolter of complete rape exists). And she is squishy as hell and locked out of the viable Power Armor options.
I am doing a full on hard dogmatic run right now with a warrior assasin main with a shield built to skill monkey, custom santic sniper op/assassin buff bot (holy fuck is that amazing), Cassie, Argentia, Heinrix, and Wulfgar in the last spot when I don't have to drag a goober for a quest along. I am not really super optimizing things and am pretty much steamrolling the game (If I had gotten the last genestealer fight prior to chapter 3 that might have been an issue) and my main guy hits like a fucking truck, with all skills over 100, and tanking that would make Abelard tuck his balls back. I took things that were good, but nothing like the pyro executioner stuff that autist built his spreadsheets around. The custom santic sniper is the only character I put any effort into really optimizing and that is purely because I wanted a buffbot. And it is still a faceroll on hard, mind you, but who would I replace in my lineup to make room for Jae?
Side note for anyone about to roll with a Dogmatic run, there is some amazing fucking synergy if you avoid ever including Xenos in the party, both in terms of stat boosts and item effects. It is insane how outrageous you can ramp up the stat synergy and resolve building by doing that. Yrliet is good, but she is not that good.