Disclaimer: I’m not a real world pilot, aerodynamicist and do not have either JF Arrow or Flying Iron Simulation’s Spitfire for MSFS yet (waiting for their arrival in the marketplace).
Yet many pilots, including commercial pilots and CFIs who tested JF Arrow and others such as Carenado PA44, are quite satisfied with the amount of adverse yaw most of these planes generate. I’ve been told by pilots that two C172s might exhibit totally different amount of adverse yaw, depending on a lot of things. Most pilots who are also simmers tell me that adverse yaw exists in MSFS, but it’s a bit on the ‘weak’ side, and should be looked into by the core sim devs. However, Carenado cracked it very well, and in their PA44 you have to apply a considerable amount of rudder to make coordinated turns, specially at low speeds. I was also watching a FSX A2A Comanche stream last night where it exhibited little to absolutely no adverse yaw during gentle turns.
The developer said, and you quoted:
Perhaps the developer needs to consult with Flying Iron Simulation developers. More on that later.
Here are some demonstrations of JF Arrow’s stall characteristics done by a real world flight instructor, commercial pilot and Q400 first officer, who also has more than 200 hours in Piper Arrows in real world (first forwarded to the point where it starts):
In brief, he says: “Stall characteristics are bang on the money. This is the exact behaviour a real world Piper Arrow would exhibit during stalls. This is the best representation of a real world Piper Arrow you could get in a home desktop simulator. It exhibits “book” numbers at all times.”.
This I’m not too sure, as the pilot in the video above didn’t do this particular manoeuvre, nor did I find any video demonstrating it. He did a 45 degree steep turn demo though, and was happy with what he saw.
In conclusion, he claims the JF Arrow is the most book-accurate plane he ever used in a simulator.
Let’s move on to Flying Iron Simulation’s newly released Spitfire for MSFS. Why do I think it’s important?
Stormbird (his pseudonym) who’s been using flight sims (specially combat flight sims) since the commodore 64 days, has flown the study level Spitfire in DCS World for approximately more than 400 hours, and has been doing reviews of various combat flight sims on his blog since 2016, had the following to say about Flying Iron Simulation’s Spitfire for MSFS after comparing it with the study level Spitfire in DCS World (and another one in IL2):
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“Takeoff run, with trim set, is more similar than different across all three Spitfires” in other sims.
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“Once cleaned up from takeoff, I can say almost immediately that this aircraft feels very familiar. It’s a type that I feel very much at home with in other sims and I had the same feeling here too.”
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“With many hundreds of hours on the IL-2: Great Battles Series Spitfire IX in pitched multiplayer and single player battles and at least several more hours on the DCS: Spitfire IX, I can say that all three feel about the same while cruising along and while using the controls in cruise. The Spitfire is noted for being light on the elevator and heavier on the ailerons and you can feel that here”
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“Landing here generally feels very good too. The Spitfire in all sims remains controllable down to near stall speeds and my almost instinctual nose up trim use came in use here as the speed bled off. There’s also an immediate nose down movement when the landing flaps are deployed and the experience felt the most like the DCS Spitfire IX. Touchdown feels like the other two Spitfire’s and is generally predictable unless too much power is applied.”
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“Roll out after landing is very similar to taxiing with this being the easiest of the three”.
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“Excellent visuals, strong sound effects, and a good flight model on the whole makes this an excellent aircraft to fly and a challenging warbird”.
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Some of the systems and failures are not simulated yet (due to limitations of the current SDK and not MSFS). These issues and limitations are very minor and “well within the (development) team’s grasp”.
In conclusion, he says that when it comes to the overall flight model, apart from taxiing behaviour on the ground, everything else in FIS’ Spitfire in MSFS feels exactly like how it feels in the DCS World Spitfire addon. Source: Review of the Spitfire IX from Flying Iron Simulations for MSFS – Stormbirds
My point here is not to say your criticisms are invalid, nor am I challenging them. Nor am I or the more than 96 percent of flight simmers who never flew a plane (according to the very latest Navigraph survey) are qualified to do so.
My point here is to present some facts, and reviews that I collected from real world pilots, and offer my two cents: Nothing in the world is set in stone. No one flight model of one single aircraft is set in stone, in either real life or a sim. Developers disagree with a lot of things, so do flight model specialists, scientists, and pilots ( two examples: https://twitter.com/JonDopplerWX/status/1370185416930959364 and https://twitter.com/aviatorwriting/status/1370990615584907265 )
The JF Arrow developer/s might need a bit more education on how to work with the new MSFS flight model and SDK? or may be they don’t (cause the flight model they developed so far is “bang on the money” by some real world pilots?) ?
Six months after release, Flying Iron Simulations develops a flight model of the Spitfire in MSFS that accurately resembles that of a study level Spitfire for DCS World? How could that be, if the MSFS flight model was so bad, and “there is a disconnect between what Asobo are seeing, and what other people are seeing, including developers with 25 years of experience.” ?
Does this mean not all 3rd party aircraft software developers are equal? 
In the words of Matt Nischan, one of the developers of Working Title Simulations:
There’s no conclusive observation that can be made by looking at one specific aircraft flight model configuration and then applying that conclusion to the entire flight simulation. How well a particular aircraft meets book values is entirely dependent on how well the flight model author adjusted the values to make the book values possible.
In the right hands, the MSFS modern engine is going to produce some seriously accurate aircraft. How do I know that? Because our Working Title CJ4 does actually hit those book values at all regimes, with correct N1s, fuel flow, climb rates, over various altitudes and ambient pressures. Not only that but we have stall speeds within a knot of two of book, proper approach angles, correct bank rates, etc.