When you first look at the Nephilim, it seems to be a fairly decent Flyer. It packs either a twin Lascannon or the Avenger mega-bolter, twin Heavy Bolters and two Blacksword Missile Launchers, giving it plenty of firepower, and has access to the usual Ravenwing rules and defensive rules for a fast flyer. So far, so good. However, since it's weapons are Heavy and it can't not move since it can't Hover, it relies on the Strafing Run rule to keep its effective BS at 3+.
Now let's look at that again in case this isn't jumping out at you- i.e. in case you either wrote the Codex, or play-tested it. The Nephilim is a Jet Fighter, which as we all know from Death From The Skies is an air superiority flyer. The GW Webstore says:
"The Nephilim Jetfighter is as an interceptor that establishes air superiority of over the battlefield, allowing troops to concentrate on ground targets with little concern for aerial assaut."
Because this whole thing annoys me, I'm going to be all Internet Debater for a moment and point out that they misspelled 'assault' up there. (Edit: Ye Gods, the more I read that quote the worse it gets. Who proof-read that?) Yeah, I know, cheap shot. Anyway, as we can see, and the Codex backs this up to by calling them "Sleek air-to-air interceptors" (P.47), the Nephilim is supposed to be for engaging flyers. Even its weapons are geared to this, with the Blackswords giving up a point of Strength to most other Flyer missiles and gaining AP instead, making them a little better against light Flyers.
And it has Strafing Run.
In case you aren't seeing the implications still (why hello Mr or Ms Codex Author, glad you could make it) this means with its Heavy weapons, the Nephilim can never hit an enemy Flyer on better than a 4+. If the Flyer is Airborne and therefore has the Hard To Hit rule, that drops to 5+. So we have an air superiority Flyer that can't hit other flyers with even the accuracy of the average Imperial Navy stick-jockey. Worse, the Dark Talon, which is already a better choice for most ground attack roles due to its Hurricane Bolters being Rapid Fire and therefore hitting on 2+ as well as having a more powerful big gun in its Rift Cannon, is actually better at hitting Flyers with those Bolters than the Nephilim. I'm not a spreadsheet guy, but I'd be willing to bet that the Hurricanes on the Dark Talon will match or even better the Nephilims entire arsenal against a Flyer through weight of shots and superior accuracy. And the Dark Talon, boys and girls, is cheaper and can Hover.
So, how do we fix the Nephilim, and does it need 'fixing'? You can probably guess from every word that has preceded this that I think the answer to the latter question is a resounding yes. You could make the case that the Nephilim as it is is a decent ground attack Flyer, and it sort-of-is, but the Dark Talon simply does that job better and cheaper. Fixing it is pretty simple:
The Nephilim should have Interceptor (+1 To Hit vs models with FLY) rather than Strafing Run.
Now there is a definite downside to this fix, in that the Nephilim would then become a less effective, or if you prefer even less effective, ground-attack Flyer. However, since ground units generally don't have a similar rule to Hard To Hit the Nephilim would at least still usually hit them on 4+, which isn't terrible for a unit engaging something that isn't its preferred target.
I've reached out to GW via Facebook and email over this, because it's patently obvious to me that something is wrong here. I'll post an update if I get a reply!