The only Alvarez characters that got nerfed and held back due to the plot IMO are Acnologia Zeref August Irene Bloodman God Serena. The other Spriggans have no excuse for their shitty performance and Glass Cannon level defense/durability.
But it's a fact that the S12 were only defeated because of plot. Without that they could easily defeat Ishgar.
Without Ultear plot appearance during that battle Dimaria would have killed Wendy and Cherria.
Without Hiros dumb writing Gajeel would be dead or at least Bloodman should survive too. But sadly he isn't a FT member.
Without the bs plot Gray and Juvia would both died against Invel but again Invel like Dimaria had bad luck that Gray's rage was stronger than Invel....makes no sense but okay.
Without Stings white ds magic, they would also died in the battle against Larcade. Most of them even Gildarts would have probably died throught his pleasure magic. Larcade is a etherion why didn't he use his etherious form? Well again a plot device for the ishgar mages?
Without Brandishs soft side she could probably killed most of them. Like I said she could easily turn her S12 comrades to big giants like she did with Gajeel too. Image a fucking big Dimaria or Bloodman.....plot strikes again.
Without Jacobs fucking virgin personality he would killed Lucy, Natsu and Mira in his battle against them. That wasn't funny what Hiro made, it was sad.
Without Hiros dicision to let God Serena be killed by the hand of Acnologia he would be a very strong opponent against Gildarts, even if Gildarts would win, he would lose a lot of magic power and couldn't fight against August. So God Serena had to be killed.
Without Irenes mother feelings Erza and Wendy would die in few secs.....the plot strikes again.
Without August fear against holder magic he would killed Gildarts and Cana. WTF I don't know what Hiro thought when he wrote that.
Without the plot Laxus probably wouldn't win against Wall, then Wall could use his attack that he used at the beginning of the war, with that he shot a 400 km long laser or why didn't he use Etherion? Plot again...
Only S12 who lost without plot or would lose anyways were Neinhart and Ajeel. But only his battle against Erza, Ajeels battle against Elfman and Lisanna was plot bs too.
Same thing with Neinhart, what was the reason behind "show you're true self" or what ever Irene said to Neinhart....makes no sense, because we never saw it. Maybe because Hiro thought he would be to strong? I don't know.