I think a lot of the issue with the knight's power is that neither of the current knights are really trying to use their power to the fullest. Harry hasn't really put the mantle through its paces that we've seen. I'll divide the mantle up into three "areas," mental, physical, and magical, and look at what Harry has done with each.
Mental:
Harry sees the winter mantle's mental influence as entirely negative, and works only to suppress it. This makes sense, given how afraid of his own darker nature he is, and the repeated theme of "Harry is often dense." Even in CD and BG when he lets it run things in battle, he's basically just leaning on the basic, hindbrain inclinations, not using it. My theory is that the only time we ever see the mantle fully in control is when he's trying to kill Rudy in BG. Harry thinks of his mantle as just animal savagery, but Mab IS winter, and she's mostly logic, not animal aggression. Pure winter is cold primal logic unbound by remorse or ethics. It's ruthless logic, not just ruthless. I'd bet Mab was leaning on her mantle quite a bit while running all her glamours during BG. We know from Molly that an extremely talented and well trained human can do like half a dozen purely phantasmal illusions at one time. Mab is doing thousands of them, AND making them physical. That's not just a matter of having more power or more experience, that straight up requires way more processing power than a bipedal hominid has. My bet is that her mantle (an the other winter mantles) can help her with cold logic, aka processing power for magic and whatnot. We don't actually know, because Harry hasn't tried to explore the mental side of his power, only bottle it up, a choice I consider to be a straight up mistake, even if it is in character. (Un?)Fortunately, Molly doesn't seem to be making the same mistake.
Physical:
The winter mantle makes you stronger, faster, and more enduring. It cancels out pain, makes you key up for a fight faster, and improves recovery time. It makes Harry about as strong, fast, and tough as a person can be. Except no, it makes him WAY stronger and faster, it has several times, and Harry doesn't seem smart enough to explore this. The dude has ran faster than a vampire and taken a hit from an Ik strong enough to flatten cars (changes). He's lifted more than a ton and jumped forty or fifty feet (both in cold days). He casually jumped off a three story castle, and went hand to hand with a titan. The mantle obviously provides WAY more than "merely" peak human strength. Harry just hasn't put together yet that what it does in the gym and what it does in actual combat are very different.
Magical:
Harry is just using winter as a battery. Hellfire? Used it for things as abstract as imagining it helping him in mental combat. Soulfire? Uses it for communication, construction, destruction, and more. Winter? Duurrrr... Winter BAD. Emergency battery only! What a dummy. Dude even notes several times that the mantle has a seriously obvious "pack hunter mode" that orients to the people around him, but it never occurs to him to put that through its paces until Ethniu helpfully sets up an opportunity for him. I get that Harry is still growing, and a theme of the later books is him being more prepared, but seriously dude, take mental inventory. The mantle is there whether you like it or not, put it through its paces. Thomas and Mab both warn him about it in their own ways.
Overall I think we've just only seen three knights wasting the potential of the mantles. Slate was an idiot, Harry has a martyr complex, and Fix has only really had any competition for the last few years. Harry's lack of diligence seems pretty out of character, but I get that he's kinda been reeling for a few years. Still, having a repeat of the Hellfire story seems lazy. Harry trust(ed) his ability to know when to stop. A freaking Archangel trusted his ability to know when to stop. Dude absolutely should have done his testing by now.