It says something when the top plane in the game isn't worth its price. That says something.

Let's talk about the few positives of the Nightmare, it's what you expect out of a high end fighter. It has good overall maneuverability but while it's good relative to the top end super fighters of the F-22A and Su-57, it does not meaningfully surpass them.

Let's start with its biggest flaw, the ammunition. The X-40 has a very tiny magazine relative to its tier. At the highest spec it can currently load:
* 18x Railgun rounds (6x bursts)
* 18x QAAMs
* 16x UAV uses

The QAAMs are, by far, the most reliable weapon for this plane. They're also available on a lot of other planes so let's move onto the unique weapons. It's also a shallower magazine than the F-22A and Su-57 (both at 24x QAAM). This is comparable, roughly, to the Su-33, a plane a quarter its price.

The unique weapons for the X-40 are the UAVs and EML launcher. Let's start with the UAVs. These are not the Raven's mini mes, these are TS based systems that will hit the target anywhere that doesn't cause damage. Against bosses like Fuscus, they will do little. That said they are decent against slow moving or stationary targets. They are also active for a very short duration, meaning they go out and return quickly so you'll burn through those shallow uses.

Now the EML Launcher is I think the the halo weapon of this. Powerful and demanding. However the plane does little to make itself a demanding platform. A railgun aircraft, at least to me, demands stability and fast traction and both are hampered by AC8s heavier flight model. Yaw especially has been affected with the weightier system making fine adjustments very difficult. The plane's yaw control meaning the differences between nailing shots and becoming Scott Norwood and missing wide right. Ultimately, outside as a sniper for straight and level targets like Tu-95s, B-52s, or maybe early on boss fight intros, the gun is useless. The 18 shots, as it fires in bursts of three, are rapidly expended as well.

The flight model leaves much to be desired, while AC-8 has put a much greater emphasis on differentiating early and late aircraft and high and low speed turn performance (and ultimately forgetting these are fighter jets and not 737s),

Ultimately, for the million MRP that you'll come out on the other side of your playthrough, Invest it in the high end real world aircraft and buy or fill out your remaining slots. I'm talking your F-15E, F-22, Su-57, or F-35. All of these are already good stock and become better with upgrades.

If I were to fix this and to bring it up to that million dollar price tag, I would do the following:
1. Deepen magazines of QAAMS to at least 36 (or 42 with upgrades)
2. Fix UAV targeting and give another at least 8 uses
3. Give the railgun a small cone to account for railgun projectile correction, maybe 1-3 degrees off axis with 5 degrees via a part
4. Update the flight model so its inertia is significantly reduced.

It is baffling to me how PA doesn't seem to want to make their super plane a super plane. It's supposed to be a little OP, a little broken. That's why they lock it behind such a high wall only when you finally scale it thinking you've gotten the power of a god, you find that it's really just a high nomad with a broken glass bottle. Oh he's on something that makes him deadly but he can't do it long and he's going to wear himself out fast.