It’s part of how Quick was designed. They have the worst damage, middling NP gen, but the most star drop. When passive stargen becomes common, they lost any function except padding on some servant’s facecard NP gain.
There’s no easy way to balance Quick because of this. If you just add more ATK steroid, then it would buff other cards as well. If you add anything below 30% Quick up, people would still turn to Buster for damage and to Arts for NP gain. So the only choice is to give Skadi over the top Quick buff and Quick exclusive crit buff. Consequently, instead of bringing Quick to the middle, it just made them extremely unstable and niche. Quick servants are either middling without Skadi, or extremely powerful with her, no in between. Skadi is like this too. She’s powerful with Quick comps, but middling outside of that.
Waver and Merlin don’t have this problem because all of their buffs are universal (despite popular opinion, Hero Creation’s critical buff is universal). Skadi shares this niche with Tamamo, in that outside of their comps they are underwhelming. Both Skadi and Tamamo have their Arts/Quick buff cut off from their kit. Tamamo’s NP is more useful in exchange for her selfish defense skill, Skadi’s NP is less useful in exchange for her useful 50% charge.
Bottom line is, if you’re asking “what else is Skadi good for?” Then the answer is the same as “what else is Tamamo good for?” Do you consider Tamamo not worth the hype? Then Skadi wouldn’t excite you either. Tamamo gets one point from me outside Arts comp due to her novelty of non-gated CD reduction. Skadi gets none because DEF down and 50% charge is covered by Waver still.
Besides, why do people hate looping so much? Isn’t that half the endgame of FGO? Deal as much stupid damage as possible? Whining about how Skadi is only good for looping is like whining about how Skadi is only good for beating half the game. 80% of the game even. The other half is stalling when the game prevents you from killing. The goal of the game is always killing. Killing now or killing later, and it just so happen that the sooner the better. Shut up Skadi whiners, being niche is not bad.