Not taking shots or anything, but just a couple things I'd like to point out about the following:
Wow, that Charisma sure is strong huh?
....Yeah, she's not using Charisma. If you look at her character page it essentially says she can gain access to a wide variety of skills via Imperial Privilege, but has the caveat that she requires a basis for comprehending and thus developing/wielding them.
The problem with this is that in order to inspire people from within like Charisma does, you need to be able to comprehend said people, something which Type Moon's Nero was characterised as having failed to do until her dying moments, and which was actually the root issue which led to her betrayal and subsequent death, with her last line "too late. This is fidelity," being a belated eureka moment where she finally comprehends things and shifts her perspective from rampant, uncomprehending (, and thus oft-times harmful... the 'singing' man.... she locked them in....,) devotion to her people, to a truer, more attentive and reciprocal form.
As such, the living Nero they're currently faced with, who has yet to undergo that epiphany, hasn't yet gained the necessary spark of comprehension to act as a basis for a true Charisma skill, meaning that her attempts to employ it would at best result in Charisma-(False), allowing her to butterfly around and corral her people with a hollow/incomplete facsimile thereof, or in the case of notable individuals, and especially that of the MC due to Shielder's Mana Defence, Nero's forceful attempts to sway him would lead Imperial Privilege's approximation of Charisma to further warp into something like
Charisma-(False) Tyranny (True), forcibly overcoming his resistance and suborning him through external means (which as best as I can figure, seems to be some form of subconscious mental interference in this case) to cause his behaviour to approximate the desired result, likely leaving Nero herself ignorant of the difference in methodology due to her skill having functioned "perfectly".
She probably did, maybe even stronger than what she has as a Servant. Remember, Servants are weaker than the original.
My second point here is that that's not necessarily how Servants work. It's more like this:
Person has their existence (permanently, or at least so barring efforts to negate such) etched within a subsection of Akasha/the Root, by having their existence become a distinct concept in and of itself, something which is usually achieved by becoming a Hero, hence said subsection of Akasha being labelled the Throne of Heroes due to them being the majority of it's inhabitants, in spite of it also containing a fair number of villains, monsters, fictional entities, or random historical figures whom also ascended to concept-hood.
As a result of this ascension into becoming the very concept and definition of their own self as imprinted upon the Root, they become a higher dimensional existence akin to a divine spirit pretty much by default, as they gain something akin to a God's Authority, but rather than governing something like "War" or "Thievery", the Spirit's 'Authority' (which is essentially another word for 'Admin Privileges') applies specifically to the 'database' of "Their Self" which contains both what they were in life (provided they weren't originally fictional) and what has been attributed as being a part of their existence.
As a result of this, the Heroic Spirit Cuchulain would be the most Cuchulain-ish Cuchulain that ever Cuchulain-ed, literally the platonic incarnation that embodies every facet of what it is to be Cuchulain, including things such as his Divinity, all at once.
While this may sound awesome however, it has the issue of not being a functional existence once you start skipping down the rungs of the Sephirot, as more and more issues start cropping up the further you are from Keter and the closer you get to Malkuth and thus materialisation. (For proof of the summoning's employment of the Sephirot see the following line of the summoning chant: "coming from the
crown, the
three-forked road that leads to the
kingdom circulate.")
As such, in order to retain your summoned Hero's cohesion while dragging them down to a lower dimensional existence, a summoner typically binds their Summoned Spirit within the confines of a Servant Class, essentially acting as the lovechild of a search engine query and a washer/dryer's lint trap, filtering your Heroic Spirit's essence of anything which inherently conflicts with the class to which you bind it.
When combined with the defined capacity limits of the Spiritual Container into which the Servant Class is usually integrated, this can lead to your summoned servant being stripped of extraneous elements to make room for further optimisation, a.k.a chucking out or diminishing skills or abilities attuned to other classes in order to make room for those more compatible with the Class etched into the container they're being summoned into, which is why Cuchulain, essentially Irish! Heracles, is pretty much crippled due to being denied the majority of his skillset, constrained by having being summoned under the auspice of a single class.. Conversely, if the Servant utilises an identical skillset across multiple classes, (*cough* Semiramis *cough*) they can effectively be summoned under the constraints of both classes simultaneously and essentially refining the resulting servant, hence the existence of the skill, Double Summon.
Similarly to the above, if a Heroic Spirit has someone metaphysically piss in their conceptual-self pool, say by writing a fictitious accounting of said Hero's existence as some kind of abomination unto God, which later becomes synonymous with the concept of said Hero, then the 'search query' of the Servant Class would fail to filter it, leading the resulting Servant to possess a trait such as Innocent Monster or the like.
To summarise the point I'm making here, while the Servant incarnation of said Hero may be weaker than the original due to things such as their Class Constraints denying them the full breadth of their abilities (which is the case for Cu, Gilgamesh and Heracles, among others), that is far from always being the case, especially with the parameter boosts from both the Class Container and from ascending to become a Heroic Spirit in the first place; the most notable example being Hassan of the Hundred Faces, who went from being Mr. Assassin with MPD, to Mr. Assassin who can manifest said MPD as a literal one man army.
Essentially, becoming a Heroic Spirit and then being distilled down into Servant form typically leaves a person's existence more.. concentrated... than they start with due to being ascended then filtered, but the Class binding itself tends to leave said Servant less versatile than they started out. As such, it tends to diminish Generalists while benefiting Specialists by strengthening their specialisation further due to it fitting under a single Class (think Assassin Emiya, evolving Time-Accel into Chronos Rose). This is why skills like Imperial Privilege or the various Wisdom skills, and Noble Phantasms like King's Order and Reincarnation Pandora, are regarded as broken in universe, due to enabling a Servant to circumvent their Class Constraints to a relatively large degree, giving them a fairly big advantage during Grail Wars and the like by letting them both have their cake and eat it.
