I don't quite remember the Necrozma special. I do recall that Ultra Necrozma didn't really do anything but stand still like a giant statue but look on the bright side about Pokemon Adventures! What the anime does great, the manga does 10 times better! Meaning we will probably see an even more badass depiction of Necrozma. It could be Ultra Beasts vs. Necrozma. Zygarde vs. Necrozma. Solgaleo and Lunala vs. Necrozma. Lillie vs. Necrozma. Necrozma vs. Necrozma. Anything could happen. As someone who is greatly interested in Necrozma's story from USUM, I am so excited for what Pokemon Adventures Sun and Moon plans on doing with Necrozma during it's epic finale. On the gloomy side of things lol! If the Necrozma anime special is as mediocre as you said it is and the Sun and Moon Adventures take on Necrozma turns out to be pretty mediocre as well, that practically means we will probably never ever ever ever EVER EVER! see any justice done to Necrozma's story EVER again lol.
I didn't mind the Necrozma Arc, we had build up to this one for a few episodes and I doubt we are going to see the last of Ultra Necrozma. (Giovanni is still out there and he wants it. His forces are still there and they are not giving up that easily.)
That being said... if your interested in a Necrozma story that might be worth checking out. I kind of did the Necrozma story myself in a fanfic that mixes S/M and US/UM together. (Necrozma was caught by Elio in the S/M canon, but his misguided attempt to help Necrozma caused him and Nebby to fight. It results in one destroyed master ball and Necrozma escaping into Ultra Space. "This is off screen stuff that is explained when I got to this." Enough motivation for our idiot hero to try and stop this... with the very real chance that Necrozma would go crazy and kill him. "I made Necrozma 90% bonkers from pain.")
This builds up to the big fight with Necrozma. The guy having a bad run in with that Darkrai from the Mystery Dungeon games, being turned into a Shadow Pokemon for a time, getting rescued by a rag tag band of heroes from another dimension, running into the Ultra Recon Squad once he is restored, getting captured by those guys and doing something to help Elio and Lillie, even in the mist of insanity. (A test from a Mew to see if Necrozma does have good in him. Too bad his insanity makes him stay a good guy for long.) Then he breaks loose and tries to arrive on Alola to go after its light, as his arrival slowly comes, the regions light is drained, until the final day comes when the sun goes into a full eclipse. (Not unlike the one you see in Dark Souls 3. )
Then that is when the climax happens and things become a chaotic fest.
Necrozma trying to go after Solgaleo and nearly killed Gladion in his mad attempt to get that light. The pain from having that light causes Solgaleo's power to be used in out of control bursts... that result in Ultra Beasts coming in... in waves. Legendary pokemon clashing against the beasts while those that can fight back do so because it is going to be a beast rush.
Then Necrozma gets forced out of Solgaleo and escapes into Ultra Megaopolis. Which results in a group chase scene that goes horribly wrong and the USUM stuff happens when the group get kicked into the Altar of the Moone on Solgaleo... while Necrozma fights Nebby. (Lunala.) Wins... and absorbs her.
Lillie and Gladion kick the things butt, Necrozma escapes back into Ultra Megaopolis. (Einsteins definition of insanity.... repeating the same action and expecting different results.) Lillie gets on a Lunala that was the offspring of Nebby and Solgaleo... then flies after Necrozma to get her child back. (Everyone else is left to fight another wave of Beasts... and two adult Xurkitree. "The ones that are the size of a mountain.")
To top this off, Lillie is 21 years old as a veteran trainer. (She is a badass in this one.) while pregnant with Elio's kids (Twins) when she took on Ultra Necrozma... and won!