I don't like Rune Knight

[Ignimortis]

Title says it all really, but here's why.

1) Theme. The theme just feels a bit tacked on, very much as if it's a class designed from the abilities it offers and upward, rather than starting with the theme and working down.
- Why is studying the runecraft of giants a Fighter thing? Inscribing runes feels more like an Artificer thing. Study feels like a Wizard or Bard thing. Creature-feature themes feel more Druidic or Sorcerous and invoking Giants and/or their power and learning, spiritually or magically feels way more Barbarian or Warlock, generally speaking, than Fighter. Skill focus, which every rune offers, is more Rogue and Bard. Just about any Class but the Cleric or Monk would be a better fit, thematically, than Fighter.
- Why/how does it study the runes of multiple giant species, from the brutish and crude Hill Giant to the sophisticated and isolationist Storm Giant? Dragonborn and Draconic Sorcerers have to pick a lane and stick to it. Why are Fighter-Giants different, except that whoever designed it didn't put that much effort or care into actually theming their subclass?
- Where do the runes go after a long rest? Just a nit-pick, really, but inscribing a rune feels like something pretty permanent. Painting a rune on, yeah, I can go with that as a temporary thing, but inscribing implies a degree of permanence. After all, how are you supposed to..."[find] the giant's work carved into a hill or cave" if they all disappear after a day? Why does the Rune Knight get to pick and choose which runes to use every day outside of "because magic" or "something something game design"?

2) Skewey Balance. It just feels...off. There's some aspects that I could easily point at as blatant power creep or niche infringement on other Classes (if there is such a thing) and then there's other aspects that I look at and wonder why it's even there. For example, offering permanent advantage on 2-4 Skill proficiencies at level 3 is very good. Probably too good. Similarly, Cloud Rune is straight up just OP for 3rd level (and I rarely make that kind of statement). The closest other ability to Cloud Runes attack redirect I can think of is the Rogue Mastermind's Misdirection at 13th level and that comes with a heap of caveats and conditions. How is it redirecting that attack? Why doesn't it have a range limitation? Can it at least have a chance of failure? Too many questions on that one for my liking. Then on the flipside, Runic Juggernaut increases your 1/turn damage (it's not even once per anyone's turn, only on your own turn) from 1d8 to 1d10. Yes, you finally get to be Huge too (which if you're interested in being so, spellcasters have been offering with Polymorph since level 7)...but a whole extra point of average damage on one attack per turn for a minute? As your 18th level subclass capstone? Steady the horses of light and dark there, we need some balance back in this subclass! Seriously? Even the Champion Fighter is getting something vaguely exciting at that level.

3) Niche infringement. This harkens back to the theming argument, but Skill focus is a Rogue/Bard thing, so why is it in a Fighter subclass? Rage is a Barbarian thing, so why is it in a Fighter subclass? Miscellaneous magical abilities and functions are kind of a Warlock thing, so why are they in a Fighter subclass? I often see Rune Knight heralded as a poster child for why X Class or subclass isn't good or lacks lustre, but if Rune Knight is the exception rather than the norm, isn't it also the problem?

I just can't take the subclass seriously. It's a jumble of mis-matched theme, abilities and power that really doesn't gel for me. It consistently seems to be called out as an outlier, both in terms of power balance and roleplaying, for good and ill, so much so that I can't help but wonder why anyone would want to play it. I don't see the appeal outside of bland theory-craft or high-op play and even then, I really struggle to fit it into a greater whole that makes the game more fun for everyone at the table.

Can someone who actually likes this subclass explain why?

While I am not a fan of Rune Knight, here's answers to your points:
1) Because if we were to exclude Fighters from anything that another class does better, Fighters would have NO niche left. Swinging big weapons? Barbarian is better. Sword and shield, tanky? Paladin. Ranged combat? Ranger, duh. Magic, any flavour? Every single caster. Skills? Rogue.
Let Fighters actually have powers other than "I basic attack, but like, a lot". If anything, we need more of that, not less.
2) 1/SR redirect isn't anything to write home about. Nobody bats an eye when Wizard gets at-will Silvery Barbs or Shield at level 18. If anything, Rune Knight active abilities aren't great overall, but serviceable, and passive benefits kind of even it out.
3) Again, if Fighter never intrudes on other people's niches, it doesn't do anything anymore. It's already one of the worst classes of the edition.

Let Fighters be cool. Let Fighters be mystical and weird. Let Fighters be actually powerful.