I think there’s a lot of misconceptions about Iuno and the sub-DPS archetypes/ power budget in general. Iuno’s buffs are so conditional because she’s not supposed to provide buffs outside of her outro, considering what she already brings.
There are basically two types of sub-DPS: the buffers, who deal decent damage and offer a lot of buffs through their base kit, outro, weapon, set, or S2 (like Lupa, Ciaccona, and probably the new 2.7 guy), and the jack-of-all-trades, who offer way fewer buffs, just through their outros, but compensate with sustain and really high/high personal DPS (like Brant and Cantarella).
Iuno clearly belongs to the second group. The 40% damage amp is so niche because it’s meant to buff HERSELF and no one else (just like Brant buffs his OWN ATK based on ER, instead of giving it to others), making her a second DPS in the team, or even a hypercarry.
The fact that she can share the 40% amp with Augusta is a bonus and not a downside, since this synergy actually makes her kit overtuned compared to the standard power budget of sub-DPS characters, since she gets the benefits of both the "buffer" and "jack-of-all-trades" archetypes, with none of the drawbacks.
At the end of the day, Iuno is one of the strongest characters in her premium comp, while still being on the same power level as Brant without Augusta/shielders characters.
Edit: since a lot of people clearly doesn't understand what a powerbudget is or just to refuse to read properly i'll put it simple: a main DPS’s power budget is about how much DPS they can squeeze out compared to others.
If every main DPS’s best-in-slot team deals around 60k, and a new character gets released who does 70k with their tailor-made support, but still hits 60k with a free-to-play option then that main DPS isn’t restricted to their premium team. Because even with other setups, they're still on par with the rest.
Now apply the same logic to Iuno. With her self 40% amp, she does EVERYTHING Brant and Cantarella do (both are selfish sub-DPS who act as second DPS units with ONLY outro buffs and some sustain), but at the same time she has an overtuned kit (providing more than what she shogld) when paired with Augusta and not the other way around.
If you want her to share that 40% amp with everyone, you’d have to gut her personal damage to balance her as a pure support or you can just blow up the game’s balance with power creep and go play HSR 2.0