It seems every single demon champion is a powerhouse on their own

Umm no, manaburn is busted, that's why demons is meta. It completely denies 90% of the games carries. You can still go rangers but rangers are trash without demon/glacial. Aatrox is over-tuned though.

the demon trait is only good on specific champions that aren't demons themselves

ashe trist lucian (im not sure how the splashing from graves auto works with onhits) voli after he ults and eve (eve is already a demon but her being an assassin gives her access to the best units you want to drain she also randomly gets to execute some champions so yea)

and when you have all 6 of them the 4demons aren't really strong 2 demons are ok if you have them on rangers ashe demon is insane imo

When Lucian stops scooting around all game, I don't think Demons are "meh"

I am not sure this is true. Except Aatrox who really is powerhouse rest of the demons are meh.

Elise - super broken because of the dmg to players, but that's about it and if you can't get her to tier3 there is no point using her at all.

Varus - very strong early, falls off late super hard.

Evelyn - she would be good but that ult of hers is stupid and loses you match most of the times. You give her no items -> she dies before building mana as she starts from 0 and she has slow attacks. Without assassin trait or against PD she barely does any damage. What is worse if you actually give her items like serapt or ludens what happens is that she jumps at important makes few autos, ults. Because she does barely any damage, she didn't get her target below execute HP treshold, her ult does nothing, she repositions and she switches aggro to different unit which gets her killed. Even if she won't switch aggro, the time she loses by need to walk back is usually what makes her to die.

Morgana - she is good vs assassins if she actually survives being oneshoted. Besides that her strong ult is hard to utilize, she will only ever damage one max two targets which is quite a waste. Kennen AOE range is 4x higher than hers. The only way I see her being utilized properly is to give her assassin spatula so she can jump into backline where she will have higher chance to damage more units.

Brand - seriously probably the worst 4gold unit in game. He will never do anything without items and because he is 4gold unit that is absolutely useless at tier1 you cannot really save items for him.

Swain - very strong but suffers from similar issue as Morgana (though in his case it is less problematic as his ult is ongoing and when he repositions it still works). However you need 3 items to make him survive which is worth only if you were given defensive items. I never got him to tier2 unfortunately as at that level he is able to 1v9. But you cannot go into a game with "I get tier2 Swain and win".

Aatroy is overtuned and they should revert theri last buff to his skill damage, Elise is garbage if it wasn't for the demon trade and her spiderlings causeing exta player damage, and the rest is strong but within line for their tiers.

The demon buff it's self however is insane, it totally fucks any non gunslinger or ranger setup, manaburn is just a disgusting mechanic in that game. Though it's true, since each individual demon already carries his weight it allows them to full make use of their utillity trait without tradeoffs, while glacial has pretty low damage overall unless you get voli fed with the right items(though you can easily supplement them with some DDs).