Yeah, we have only seen FSS as an learnable example but pure spirit arts seems to be stronger then what would be expected of their source (like Zeqing). But this seems comes at a cost, that it influences the person that learns it more, , mind it might just be an aesthetic thing.
Which might make sense, considering that spirits are their Domain/Way from birth/creation rather then become it like humans do and a cultivator is effectively learning and maybe incorporating a piece of them. A few arts we seen talks more about copying what a spirit does which seems safer.
Think its more primal than potent.
Like, what little we've seen suggests something like:
-Spirit arts are literally a bodily function for them. They do their One Thing ridiculously well. For Frozen Soul Serenade, its the concentrated power of cold of the spirit of a blizzard. It Brings An End.
-Human made arts tend to approach one or more of three roles:
--Express a philosophy or insight
--Describe a phenomenon or spirit
--Fulfill a tactical or strategic role
And what we've seen is that at lower quality, a human art usually does one of the three, but at higher quality, it can perform all three to a high extent, as epitomized by the Ducals. We've also seen that they can fit into other techniques like puzzle pieces for a squad or army level technique, though Ling Qi has never had to do that before.
While at the same time Spirit given arts can be awkward to use with other arts, because they're structured around assuming you have other features of the spirit.
Take Zeqing as our best example:
-Frozen Soul Serenade sets up a small zone of cold, then requires you to get in close after priming the pump to annihilate their defenses in a single catastrophic blow. It doesn't however, help you get in close or land that melee hit needed to maximize effectiveness, nor does it have any built in strategy for finding a way to prime for the Call To Ending while not getting splattered.
-Lonely Winter Maiden incapacitates the target and lures them to the Yukionna, but it doesn't actually have a finishing plan on its own, they're debuffed and obssessed with you, but using it alone to drain a peer or higher opponent just means you've lured a foe into melee without defenses or any strong melee attacks.
-We hadn't seen the original version but we know theres a Spine/Leg art for them, and Hanyi's version is Fleeting Spring's Chill. It gives dodge, the speed to taunt a fast opponent without them closing faster than you'd like, and it gives perfect defenses for melee use. Based on how Hanyi used it back in Yellow we can also surmise that it lets you run on vertical surfaces and possibly dance on falling snowflakes.
-And we hadn't seen any mechanics for it, but its likely they have an AoE zoning blizzard summoning effect that applies whiteout conditions, allowing the two offensive arts with their single target preference to pick one target and isolate them to focus down.
Put all four together and you have a potent art suite, but they don't play nice without some kind of similar strategy in your build. A spirit art may instead give you jaws the power of a crocodile, but not the natural armor or disguise to effectively apply a bite force capable of shattering anything it can get between its teeth.
The big challenge of high nobility seems to be that they have an art suite like a complete complementary weapons system that supports a strategy coherently. Their arts are more like having a suit of armor, a lance for charges, a mace for armor, and a sword to cover the things that don't fit the rest. You could swap out one piece for another depending on your foe, rather than build around a specific core conceit. You could lose a piece entirely and still function, suboptimally but you can fight.