The thing about Squad Zero is they kinda break the power scaling in a way that's frustrating, but maybe that's the point. They're introduced so late, and they're supposed to be these ultimate protectors of the Soul King, way above the Gotei 13. But their actual impact on the main plot feels... weirdly minimal and maximal at the same time? They show up after the first invasion of the Quincy and basically whisk Ichigo and Renji away for power-ups, which is a major plot device. Without them, Ichigo doesn't get his true Zanpakutō, and the final battle couldn't happen. So functionally, they're a massive deus ex machina.
But then in the final arc, when the Quincy invade the Soul King Palace, they all get defeated pretty quickly by the Sternritter. It undermines their whole 'ultimate' vibe and makes you wonder why they were built up so much. Their main effect is stalling for time and revealing lore about the Soul King, which shifts the entire moral grounding of the series. The plot becomes less about saving Rukia or Karakura Town and more about the creepy, hidden foundation of the soul society itself. They're a narrative pivot point, even if their fight scenes are anticlimactic.
Honestly, their biggest legacy might be the backstory they dump about the original sin of the Soul Reapers.