True enough. It's also worth mentioning that the Stun only happens on HP break, so that might not actually be a skill of hers. She also has full NP charge on break, but so did Valkyrie and Sigurd in their boss fights so I feel like I can safely rule that one out.
I did mention the crit up only affecting Quick cards, but I don't mind it to much. Especially considering that she has 50% Crit up on NP.
I was mostly sticking to people who can give Quick up to other people. Tristan is more of a general support who happens to have a Quick NP.
Only because Quick has been lacking in support options for three years at this point, whilst Merlin has been out for a year and Tamamo even longer.
I would argue that this is the big problem; the barrier to entry.
The complaints about playstyle I don't think really matter due to the fact that this game has no PVP function, not to mention that the PVE in this game rarely ever needs an 'optimal' playstyle. I care only minimally about the inherent weakness of Quick cards because that sort of thing only matters in like 0.01% of the situations you could actually run into in this game. When I say 'I want a proper Quick support', it's not because I'm obsessed with Quick becoming as viable as Buster or Arts (not that I'd be opposed to or support some QoL changes), it's because my two favourite servants, amongst many others I like, are Quick-based and I want to be able to use them without feeling like I'm handicapping myself to do it, and this is coming from an unashamed tier-whore.
I mean, that's sort of true. I won't deny that a lot of Quick supports in the current meta are defined by being self-reliant, but I'd also argue that a lot of Quick Servants suffer from having outdated kits. To continue with Okita as an example, your description of Sigurd describes her perfectly as well; capable of murdering almost anything but:
-Lopsided and overly-bursty damage output
-Minimal sustain hampered by being tied to DPS skills
-Minimal to non-existent team utility
-Falls off hard against enemies with added gimmicks and additional health bars
I'd argue that the 'dated kit' label applies to most Quick-based servants except Jack, Kintoki, and maybe Scathach and Atalanta Alter, but it doesn't change the fact that while Arts and Buster get a wide variety in their attackers, almost every prominent Quick-based attacker deals solely in huge, 1-turn NPCritCrit chains with little to non-existent team utility.
Again, I'd put that down less to Quick cards being weak compared to the other two, because as I've said I don't think that really matters, and more because Quick teams haven't been given adequate support by DW.
Lets use Arts as an example:
-Tamamo as the primary support (NP Drain, Heal on Skill and NP, 50% Arts up for 3 turns, a hefty, party-wide NP charge refund on NP, skill cooldown-reduction, Shapeshift for sustainability)
-Jeanne as the Queen of Stall as a secondary support (Crit stars per turn, Stun, Invinsibility, Defense increase, and HP regen on a somewhat spammable NP, ungodly sustain thanks to highest HP in the game combined with Ruler resistance)
-Nero Bride (Targetable NP gen up, attack up, star gen up, HP recover, def up, and burn + def and critical damage reduction on NP) and Sherlock (NP seal, gain stars, def reduction + party wide ignore invincibility and def on NP) who are capable of fulfilling a supporting role whilst being viable attacks in their own rights.
-For huge burst attackers, you've got Artemis (Absolutely shreds male enemies + attack reduction and chance to reduce NP charge on NP), Vlad (NP drain, sustainability in Def increase and guts, gain stars on NP, near-universal attacker due to being a berserker), and Da Vinci (NP Damage up, Guts, Def increase, Debuff immunity, HP recovery, Charge up per turn, Ignore Invuln, Gain Stars)
-For attackers with more utility, you have Kiara (Arts resistance down, Debuff resistance down, NP gauge down, reduce def, effectiveness against Rulers, improved def against humanoid enemies, invuln pierce and ignore def buffs on NP), Archura (party-wide def boost, HP recovery, party-wide atk up, increase star gen for male allies, change for NP gauge down on NP), Houkasai (debuff resistance down, def reduction to enemies hit by arts cards, 2 stars every turn from passive)
That's a lot of options for team-building, and what about Buster?
-The best support in the game (Merlin)
-A good secondary support with godly team sustain who can also put out good damage (Nightingale)
-The best burst-attackers in the game (Gilgamesh, JAlter, Quetzalcoatl, Musashi)
-The best wave-clearer in the game (Raikou)
-Attackers that also have amazing utility and team support (Amakusa, Ozymandias, Ivan, Ereshkigal, Semiramis, Brynhildr)
-Attackers that have the best survivability in the game (Cu Alter, King Hassan)
Comparably what does Quick have?
-Osakabehime, who whilst I think has potential as a secondary support, is nowhere near good enough to carry an entire team by herself like Tamamo or Merlin
-No primary support as of time of writing
-A lot units specialising in one-turn burst with minimal or non-existent team utility/sustainability (Okita, MHX and MHX Alter)
-Some attackers that do have utility, but still specialise in one-turn busting and/or are held back by other problems with their kit (Scathach, Jack, Meltlilith)
-A mediocre AOE attacker (Dantes) and an AOE attacker who has some good team utility (Achilles)
And this is looking at 5*s only, if I were to expand it to 4*s the disparity would become even worse.
My overall point being that yes, Quick cards do have inherent disadvantages when compared to the other ones, but the gameplay of FGO as a whole means that those disadvantages in the vast majority of situations don't matter at all; the reason Quick is regarded so poorly by the fanbase has more to do with the fact that DW hung it out to dry and gave it almost no tools to work with, especially compared to Buster, the card type I would go so far as to argue DW is bias towards and deliberately pumps up because they know people are more likely to whale for Buster units because of the Dick Wizard.