Side note, do we use the rles/profiles of old weapons or new ones with these things. This is relevant for the warp hunter too because i don't wanna pay 35 points for the new holo fields.
Unfortunately, you use the upgrade prices in each unit's entry, so the Hornet and Warp Hunter end up paying extortionate prices for some things. Fortunately, even cheap Holo-fields would be a bit extravagant on the Hornet, and the Warp Hunter has a Barrage weapon, so it's not a big deal.
The Hornet weapon pricing is going to have the biggest impact on when they will be useful. Consider the following:
Hornets vs. VypersVyper w/2 Shuriken Cannons
Hornet w/2 Shuriken Cannons
Exactly the same firepower, and the Hornet pays 5pts for a closed top, Scout, Star Engines, and AV11 on front and side. No contest; replace the above Vyper configuration with a Hornet in all lists.
Vyper w/Scatterlaser, Shuriken Cannon
Hornet w/Scatterlaser, Shuriken Cannon
Same situation as the previous example; for +5pts, take the Hornet every time.
Vyper w/Eldar Missile Launcher, Shuriken Cannon
Hornet w/Eldar Missile Launcher, Shuriken Cannon
Take the Hornet for +5pts. However, the Hornet isn't stuck with that Shuriken Cannon, so let's make another comparison.
Hornets vs. War WalkersSince both models have Scout, and the Hornet's Jink save is roughly equivalent to the Power Field on the War Walker, for all of the weapon configurations in this section I'd happily pay the 5-10pts for the AV11, Star Engines, and Fast Skimmer movement. However, this comparison is a bit trickier, since the War Walker has Battle Focus and Fleet, so it is theoretically more difficult to engage than the Hornet, and it has the option to assault if necessary. Choose the unit for the role you need it to fill, depending on your available FOC slots.
War Walker w/2 Shuriken Cannons
Hornet w/2 Shuriken Cannons
War Walker w/Scatterlaser, Shuriken Cannon
Hornet w/Scatterlaser, Shuriken Cannon
War Walker w/Eldar Missile Launcher, Scatterlaser
Hornet w/Eldar Missile Launcher, Scatterlaser
The Hornet starts to lose appeal compared to the War Walker when you start trying to put Starcannons or Brightlances on it. Those aren't the only high-strength, low-AP weapons the Hornet can use, though, so the following comparison might be relevant.
Hornet vs. Crimson HunterHornet w/2 Pulse Lasers
Crimson Hunter w/Pulse Laser, 2 Brightlances
If you take Crimson hunters to solve your flier problems, the Hornet cannot replace them. However, if you don't face many fliers and use your Crimson Hunters to crack open vehicles instead, replacing them with Hornets is certainly something to consider at -35pts.
Remember: the choice to field non-Apocalypse Forge World units is your choice, not your opponent's choice. Few Forge World units are actually better than taking Codex units, and those that are better aren't powerful enough to be called unfair. For example, compare the Hornet to the Tau Piranha and you'll see that it's reasonable. if your opponent complains and/or refuses to play against you, 1) they probably weren't worth spending time with in the first place, and 2) they should be whining to GW, not you, because you can't be faulted for using the best tools available.