I don't like Rune Knight

[JellyPooga]

Sure, I can do it.

Have at it!

Hard disagree.

Please explain why.

Runes are a system, a language, and one linked to all Giants, their gods and their Ordning. You study the language itself, and the past & present of the Giants.

The Fighter is basically doing the equivalent of a Symbol spell.

Ok, I'll bite/concede on these. It's intriguing/engaging enough that I'll let it go as a sticking point, at least.

It's powerful, but it's once per long short rest. You have to use it carefully to make it "extremely strong" as opposed to "nice, not getting hit this one time".

Fixed it for you. There's a big difference and unlike the Drunken Monk, it works on a hit, not a miss, which is crazy better. It definitely needs toning down or level gating.

There is no such thing. Fighters can be good at skills, can have miscellaneous or even powerful magical perks, resist damage, and so on and so on.

Are you against the Eldritch Knight? The Psi Warrior? The Battle Master with the Maneuvres that improve skill proficiency?

You'll note that I questioned whether niche infringement even exists myself and no, I'm not against the likes of the Eldritch Knight any more than I am the Bard, Druid or any other Class or subclass that occupies multiple roles. What I'm really questioning is why the Rune Knight spreads itself so thin, thematically and mechanically, across a spectrum of other niches when had it occupied that niche in the first place, would or could have been so much more than it is.

For example. Let's run with the Rune Knight as a Barbarian subclass instead of Fighter.
1) Barbarians are a specifically Strength themed Class with strong ties to clan culture, tradition and savagery. Slotting Giants into this trope is not difficult by any stretch of the imagination. Runes and their use, both in divination and in markings/tattoos have a solid tie to shamanistic and "barbaric" cultures. Theme is 100% on point here.
2) Then there's the Runes themselves. Does it matter that runes have to be on equipment? I mean, it's only a minor limitation given that you inscribe runes every day. Why not just make them a more generic feature, fluffed as whatever you want; woad paint, etched in your armour, tattoos, ritual scarification...you do your runes however you want; open up the theme to more than just inscribing runes on their arms&armour. Thematically still on point for Giants, given the broad range of giant cultures our PC could have learned from and it gels with Barbarian nicely.
3) If we're running with Giants as an ancient culture, let's add a little sophistication to it; we already have our "Roid Rage get Big" feature, so let's look at other giant stuff for our runic inspiration. Storm Giant Divination? Yeah, cool. Throw it in for a shaman feel. Crafting with Fire Giants? Heck yeah, no-one wants to wear heavy armour in the forge; get that naked Barbarian schtick going! Prefer chillier climes? Frost Giants baby! Only the gnarliest of Cimmerians tackle those frigid wastes. You see how the different Runes/Giant types could play upon and enhance already existing Barbarian themes?
4) Then there's mechanics. Specifically Rage. Hey, we already have a mechanic we're utilising in this class that offers bonus damage and advantage on strength checks. Great. Let's make it better by offering a size increase (maybe even more additional damage) and perhaps even some scaling by level or on-choice elemental damage from different Giant types. No need to add a mechanic and a pool of depletable resources to a Class, when we have a Class that already uses that mechanic.

edit: Ok, so I only just found out that Path of the Giant Barbarian is a thing. I still think Rune Knight fits Barbarian better thematically and mechanically.

edit2: Giant's Might is definitely akin to adding Rage to Fighter. Nitpicking details doesn't make it less true.