I think Ho-oh GX is pretty underrated. It's not like we don't have any decent Fire acceleration. (Baby Volc, Max Elixir, Turtonator GX)
180 is an OHKO on most basics. Add a Choice Band or a Steam Up and we OHKO most Stage 1s. Add both or 2 Steam Ups, and we OHKO almost everything.
Alternatively, we can give Ho-oh a FFB, boosting it's HP to 230, and swinging for 190 (OHKO on all basics except Wishiwashi GX and Wailord EX in expanded). One Steam up gets us 220, 2 gets us 250, the current HP cap.
Heck, in between Phoenix Burns, we can soften up problem Pokes with Sacred Fire.
The GX attack is highly dependent on whether or not we get a good Charizard GX (or another fire type GX in the future), though, Volcanion EX probably wouldn't mind the free search.
Finally, weakness to Lightning makes Ho-oh a good tech in Volcanion just to counter Water Box, Ninetails, and other Volc decks.
The only way I see Necrozma GX working is with Metagross GX, unless we get some new Psychic Energy acceleration. Sure, it can wall Mega Ray, Drampa, and Tauros, but Mega Ray plays Hex, and Drampa and Tauros are usually partners for other types.
I like your optimism towards the new Ho-Oh~
I was thinking about trying it out in Volcanion, but it seems like a "win-more" card.
The reason I say this, is because:
Volcanion already doesn't have trouble OHKOing.
130 + Choice Band + Steam-Up = 190
Which, as you stated, will kill most basics.
What's important to think about, is what it means for Ho-Ohs attack to cost 1 more energy.
This is an additional attachment or energy to accelerate.
That's one turn behind, and/or one less steam-up.
With that one energy, you could swing for 220, instead of 190... which... I think kills anything that's relevant in the meta, except Lapras + FFB, but that's just one additional steam up on a card that's already kind of slow setting up.
Especially since Decidueye and Solgaleo are already weak to fire.
and there's almost no situation in which I would want to use Ho-Oh's GX attack over Turtonator's.
Especially since:
1. Pokemon Stretcher is a thing.
2. Oricorio does the same thing, but it's from the deck, isn't limited to EXs and GXs, and doesn't use your once per game GX attack.
There's already a lot of cards competing to get into the Volcanion deck (ex. Starmie), and I just don't see this card offering enough to warrant a spot.