May I ask you? Why you think 9G should be stronger than OS and GH? Only bc they were after them?
Yes, some demons are really strong. Mard, Seilah, Silver, Kyoka. But others? I don't think so. They never show any impressive power.
Franmalth just immune to magic a little but can be oneshotted by casual non magic\human strike. Shot him with small rock and he will be done.
Keis was killed by water blow, As fighter he is cool, but his actual lvl... no, he is not a strong character.
Ezel. Physically strong enough to fight Gajeel or same mages, has high speed and penetration damage. Ezel is cool. But is he stronger than Azuma, Bluenote? I don't think so. Is he stronger than Cobra was in past? Maybe, but no so much. Is he stronger than Cobra in their arc? No. He was defeated by Wendy's attack (really strong attack, but this is still Wendy). Don't forget, FT's main party didn't up too many times, they are just got SO which means they become 2 times stronger.
Torafusa has no feats except his underworld. His attack power not so good, his defense without his curse water also not so good. He is just mid tier fighter. He stronger than more of OS and GH, but he is just physical fighter. What can he do against Rose, Ultear or Midnight?
Jacal has high "bomb" power. I think his lvl around Jura's in 7 year past. He really dangerous fighter bc of his curse and attack, but he is not so strong as himself. I mean Azuma, Cobra or also somebody without their attack magic still strong enough to defeat more fairies.
Tempesta is the same as Jacal. He has same feats. And both of them were trash against Natsu. They didn't make a single punch on him (maybe 1 or 2, I don't remember full fight with Jacal).
All what I say is most of Tartaros were much less trouble for FT than OS and GH.
Mard, Seilah and Silver are stronger than most of OS\GH, but comparing others demons to them they more weaker, than low tier members of OS\GH weaker than strongest. (Hope I correctly write this)
And also, really many people think Tartaros stronger than OS ans GH only bc they were later? Avatar were later, so?