Hello
So it is quite common for Unearthed Arcana sub-classes to be a bit too good, and nerfed a bit when published in an "official" book. And the Rune Knight was too good - the combo of the hill giant rune giving your damage resistance, and the +1d6 dmg to all attacks was just... a little bit much. So a nerf was clearly necessary.
I did like the mechanics and the "flavor" of the subclass a lot. So I hoped it would be published officially. I wondered how it would be nerfed however. Would it be OP (probably not), just right or over-nerfed?
Now that I see the official version I'm a bit... torn. Level-gating the hill giant rune makes sense, but it's a long weight for that subclass feature to kick in. And limiting the extra damage of giant's might to 1d6 per turn, with a slow increase to 1d8 and then 1d10.... it's a bit harsh.
It's clearly still a playable subclass! But... did they go too far?
Like some other members, I never read the previous version of the subclass, just the Tasha's Cauldron version. From my point of view even in that form it is the fighter subclass that is the strongest. From my point of view I don't see why I'd choose a battle master over a rune knight except for narrative purposes, but if I just consider raw power, rune knight is way way stronger.
Most runes provide a rather strong passive bonus, and for those who say many passive bonuses aren't combat related, that is exactly my point. With fire runes you suddenly become specialized in any tools you are proficient with, for races that do not have infravision the stone rune provides you with it and makes you suddenly more insightful. From my perspective the runes can be used to either amplify your strengths a lot or compensate your weaknesses, proficiency wise. That is not too powerful in and of itself, but it is a very potent bonus. I don't see any other fighter subclasses that provides two of these kind of bonuses from the get go (at level 3). Then there are the more powerful powers of the runes. The stone rune (which you can get at level 3) gives you more or less the Hold Monster ability once per short rest, without concentration. The fire rune, while a little weaker on the hold part will both restrain and inflict continuous damage every round, again without concentration.
The DC is purposefully set to a fighter friendly stats (constitution) so it doesn't force you to at least take a more magical stats to have good DC.
Now that is just two "beginner" runes, you also get the ability with a bonus action to "weak rage". Now, the way this rage is setup, you do get diminishing returns as you level up since the bonus damage is only once per turn and not on every attacks, but until you get 3 attacks per round it is still a pretty nice bonus that you can trigger a number of times equal to your proficiency bonus per long rest and lasts for 1 minute.
Let's compare this to 4 uses of battle techniques per short rest among a pool of 3 techniques for a battle master. There is no battle master technique that can incapacitate an enemy as well as the stone rune, and no technique compares damage-wise to the fire rune.
From my perspective, the rune knight is just on another balance chart. If I ask my players to build the strongest 3rd level fighter, mechanically (using only the subclasses from PHB, Xanathar and Tasha) they all make a rune knight. Now, if we try this with an 18th level fighter, maybe there is a little more wiggle room for some other sub-class, but I am really not sure, at 18th level you'll have the use of 5 of the 6 runes and Storm and Hill runes are very strong, also at that point you can use the runes "strong power" twice per short rest... so honestly I just don't see how the rune knight is equal to other fighter sub-classes, maybe if you accept all UA subclasses it is comparable to some of those, but not to the subclasses in the PHB and Xanathar manuals.