As a card-carrying Rune Knight hater, let me count the ways...
- it's a fighter subclass centred around magical abilities rather than fighting
- it's got nothing to do with runes, and nothing to do with knighthood
- D&Ds elemental-themed giants are boring and uninspired despite the enormous effort 5th ed has put into trying to convince people otherwise
- it's silly that Rune Knights can get giant-based abilities that giants themselves don't have
- if WotC wants PC giants, they should stop being cowards and write them up as a race, not mess about with subclasses that turn PCs into part-time quasi-giants
- the 'runes' are a poorly balanced and unthematic mess. The Hill Giant ability is probably the best, and that's not the way it should work given they're the whipping boys of giantkind. The abilities the runes grant are really just a random grab-bag of buttons to press with any sort of in-world thematic coherence a distant afterthought.
- the class feature where your PC grows permanently by a small amount is pointless and meaningless and is the sort of thing that should be left to players and GMs. Possibly the only dumber class feature in the entirety of D&D is that monk subclass where as one of your class abilities you get ... a mask. Which does nothing. Except it's a mask. That you can wear on your face.
- the illusion of choice - you choose the runes you know, but there are so few and so many of them are level-gated that basically all Rune Knights will end up roughly the same anyway
As someone who's played a RK and had a great time:
1: You are still a fighter. You have abilities that make you bigger and better at grappling, do more damage, resist more damage... and a lot of the "magical abilities" are protective, allowing you to defend you party better -the RK is probably the best
tank amongst the fighter (better than the cavalier). It
is a "magical fighter" - same as the EK - but with a more primal feel, less "I spent a semester at wizard school".
2: The Runes are in the fluff. It's just that though - it could have been special crystals, alchemical extracts, etc etc. BUT having Runes that work in a
mechanical way is quite challenging. As far the knight... that complain is valid for a lot of subclasses.
3: Giants are boring? Eh, they could be better, but I played in SKT and it was interesting.
4: That is an
excellent point - and I give rune powers to giant "leaders". You know, to make them more interesting.
5: Balancing giants as a playable race is a nightmare. How many hp do you have at level 1? 3.X tried (somewhat) and it was wonky.
6: Could the runes be better? Yes. Could they be more balanced? Yes, but that's why level gating some of them was necessary I suppose. It is strange that the hill giant is so strong but here we are.
7: It's a ribbon feature, calm down
8: Once again, a valid complaint that is valid about a number of other subclasses...