I think what you're demonstrating with this character is that:
a) If you put 4 feats in to Skill improvement instead of anything else, you'll be good at the skills pillar of the game. You could pick any other class, pick up the same Feats (sans 1 for fighter) and be in nearly the same boat.
No that is absolutely not true because the
advantage on 4 skills (5 at 10th level) and the
expertise on four tools comes from the Rune Knight subclass explicitly, not from the feats.
For example take an Arcane Archer and do that same background and exact same feats and here is the differences between these two:
AA has no proficiency at all in Smiths Tools and the Rune Knight has expertise in this.
AA has proficiency in Thieves Tools RK has expertise in Thieves Tools
AA has proficiency in Cartogrpahpers tools, RK has expertise in Cartographers Tools
AA has proficiency in an instrument, RK has expertise in an instrument
AA has no Advantage in Deception checks like the RK does
AA has no Advantage in Intimidatation checks like the RK does
AA has no Advantage in Animal Handling checks like the RK does
AA has no advantage in Slight of Hand checks like the RK does
AA has one more regular proficiency in Arcana or Nature (however the Rune Knight above gets expertise in many Nature Checks through Cartographers tools and will have advantage on Arcana checks at 10th level).
AA has either Druidcraft or Prestigitation Cantrips
The other subclasses you posted are similar. 1 extra skill and maybe 1 expertise and losing advantage on 4 skills (5 at 10th level) and losing expertise on 4 tools, and potentially losing a skill proficiency
In the social and exploration pillars, the other subclasses you posted would actually be roughly equivalent to the Wizard you posted earlier.
I think when people are talking about 'a class being good at skills' its in the context of 'while also doing what the class would naturally want to do anyway'. We say a Sorcerer is good at social skills because they naturally want to max their charisma. We wouldn't say a sorcerer is good at Nature checks because they can technically dump CHA, pump INT and choose Skill Expert.
But there is a huge difference between being awesome at the social pillar (like the RK I posted) and able to
"only really contribute" to the combat pillar. My post was a specific reply to someone who claimed that the class could not contribute. What I posted was a build that is dominant.
I could have put a 14 in Charisma that took no feats at all and I would still be "good" at the social and exploration pillar because of the advantage and the expertise. I would certainly be able to "contribute" while being awesome in combat as a fighter.
The advantage is something that comes with the Rune Knight subclass. Slight of Hand and Deception advantage for exampel come with Cloud Rune, which is a pretty powerful combat Rune in its own right. Those things are part of the subclass design. The build I posted above specifically and purposely plays on these things which come from the subclass.
Moreover the example above did not dump Dexterity. I had a 16 Dexterity. I think Sorcerer example is a poor one, because even if you took feats like the RK did you would not be as good. Even with a 20 Charisma you would not even be as good at the social pillar alone unless you really drove your spell selections for that specifically (Charm Person, Suggestion, Detect Thoughts etc). Would you be
"able to contribute"? Sure you would.
Rune Knight seems like a clear standout here. You couldn't make the same claims about other fighter subclasses (unless you're feat dumping again).
Sure, but this seems to be different than what I thought you were saying earlier.
The Rune Knight is available to anyone who wants to play a fighter and it is one of the best platforms for a skill monkey. RK along with the Fey Wanderer Ranger, Soul Knife Rogue and Scout Rogue are the top skill Monkey subclasses in the entire game. Those 4 subclasses can dominate the social and exploration pillars if you build towards that purposely and do it better than any others can do while also remaining viable in combat.
Moreover the Rune Knight is really good in combat as well.
If you want to choose a different subclass then sure you will not be as good (in any pillar probably), but it is not like the fighter does not offer a platform to do this.
I generally think its a very well put together Subclass; but the giant/magic flavour is too strong for me. I wish some of the non-magical Fighter Subclasses had the same versatility. (also. I hadn't realised just how many magic flavoured fighters there were. Dammit WOTC, why can't you make a good non-magical fighter?)
I don't think people really want a non-magic flavored fighter that can do "great things". People I play with love the flavor of Echo Knight and Rune Knight.
A subclass that did those things without being about Giants, or about a phantom going around but scored the same kind of damage, skill and mobility bonuses would not be as popular I don't think.
c) I wouldn't say its a particularly competent combat build though. Attacking twice with a Scimitar + Duelling you're about 20% behind a 'baseline' Eldritch Blast warlock, or 45% behind a more typical GWM/PAM build.
Well a Scimitar is not the best finesse weapon. If you look at a Rapier and Dueling you are doing 12-26 damage with your stick plus 1d6 when using Giant's might. A baseline EB Warlock with a 20 Charisma and agonizing blast is doing 12-30 which is about 10% better. That is not including the Fire Rune.
PAM/GWM is difficult to rate because you are using your bonus action and AC comes into play. If you consider giants might, 16 AC, 4 round combats and 6 combats a day with 2 short rests, my guess is GWM/PAM will come out about 25% higher. Shorter combats, higher AC, fewer combats or more short rests will tilt that difference further towards the skill build and away from the GWM/PAM. I will do the math on this tomorrow perhaps, I need to go play in about 10 minutes.
Edit here is the math:
8th level RK with a 20 Strength GWM/PAM against a 16 AC:
Base attack damage per attack action: 17.2
Bonus action damage per bonus action attack: 7.125
GM damage on turn it is activated: 2.24
GM damage on turn with a bonus action: 2.744
GM damage on a turn with a bonus action and action surge: 3.2
In 24 rounds of combat this character has:
27 attack actions --> 492.754
18 bonus action attacks --> 137.7
3 rounds with GM and no bonus --> 6.72
3 rounds GM with action surge --> 9.68
6 rounds with GM and a bonus --> 16.5
Total daily damage -->
625.55
8th level RK with 18 Strength Dueling and Rapier against 16 AC:
Base damage with attack action: 13.05
GM damage base: 2.94
GM damage on turn with action surge: 3.41
In 24 rounds of combat this character has:
27 attack actions --> 352.35
9 rounds GM --> 26.4
3 GM action surge --> 10.23
Total daily damage -->
388.98
So the dueling guy is down 38%, which is more than I expected, but still quite fine damage. This does not consider the Fire Rune damage.