I think there's still merrit in running 1 copy of DM Necrozma in Newgaleo GX, if the deck has space, simply as a basic that can still OHKO everything in the game with a choice band.
Kind of like how Newgaleo GX wants to play Solgaleo Prism Star and Rayquaza as back up energy acceleration options that at least keep your deck operational when Newgaleo GX gets KO'd, having 1 copy of DM Necrozma gives you something you can just slap on the bench and accelerate energy onto right away with a much better survivability rate than slapping down a Cosmog under certain circumstances.
It's also helpful for whenever you prize a SuMo Solgaleo GX and your only copy gets KO'd but you still need some form of potent offense.
The Newgaleo list right now runs the 2/2 split, which so far doesn't seem to be a big problem, but adding in a DM Necrozma really does act like a 3rd main attacker.
Attackers:
2 SuMo Solgaleo GX
1 DM Necrozma GX
Accelerators:
2 Newgaleo GX
1 Solgaleo Prism Star
1 Rayquaza
Let's also not forget that under the right circumstances, if you Sol Burst GX onto a SuMo Solgaleo GX and commit some of the energy to DM Necrozma GX, you can get 2 OHKOs back to back. That 4 prize burst could win you the game.
Example Scenario:
Evolve up into SuMo Solgaleo GX, have 1 energy attached to it.
Sol Burst GX to put 2 energy onto SuMo Solgaleo GX and 3 energy onto DM Necrozma GX.
Next turn, attach 4th energy to DM Necrozma GX and Sunsteel Strike (Choice Band if needed) for OHKO. Alternatively, you can Ultra Road that turn and KO with DM Necrozma GX if you don't want to put Solgaleo GX at risk.
Next turn, Ultra Road and grab another OHKO with whichever one you didn't attack with last turn (attach 2nd Choice Band if needed).
During all of this, you should already have been digging for your Solgaleo Prism Star or Rayquaza to get your energy back.
I think the #1 thing to keep in mind is that in the mirror match between Newgaleo GX decks, Newgaleo GX really doesn't benefit you at all. A bit of an overstatement, but let me explain. Solgaleo and DM Necrozma are going to OHKO everything you have anyway, making the no weakness ability pretty moot. You would much rather go full on offense instead of try to tickle your opponent with 2 hit KOs. The Newgaleo GX is ultimately here to get rid of metal's auto loss to fire decks while providing good energy acceleration everywhere else. Let's not forget that Sol Burst GX is still the best energy acceleration the deck has to offer and it can get all the energy it needs in the mirror match without ever going into Newgaleo GX a single time (thanks to Solgaleo Prism Star and Rayaquaza/Registeel taking care of energy acceleration for you).
As for DM Necrozma/Magnezone, it has the same problem that Tapu Bulu/Vikavolt has.
If your Magnezone gets KO'd, your DM Necrozma pretty much becomes useless by default, and Magnezone isn't nearly as consistent as Vikavolt is for energy acceleration, so it's a lot more frail.
The Solgaleo GX line up not only offers nullifying fire weakness to prevent that, but also 250 HP to make sure it can't just get picked off by any attacker in any deck.
