More like you have no idea what you're talking about and just assume things based on your limited knowledge.
Here's some (generally regarded as credible) third party sources so you cannot blame me for any bias here:
(taken from altered-time.com)
I cannot directly link the image here so I'll just refer you to
https://docs.google.com/document/d/1...yZW4A/preview# page 10.
Fire's #1 legendary is at ~11% and comes at the loss of mobility (might end up slightly worse in practice)
Fire's #2-4 legendaries are at ~2-3% and come with situational benefits (additional burst phase, mobility, AoE) that might end up slightly stronger in practice
Balance's #1 legendary is at ~10% excluding trinket snapshotting (so better in practice if using trinket snapshotting because it allows you to snapshot both empowerments)
Balance's #2-#3 legendaries are between ~7% and ~9% each and come with situational benefits (free cleave, additional burst phases and more AoE) that might end up slightly stronger in practice
Now feel free to explain to me how my assessment that each of the specs has one very strong legendary but balance's second strongest legendary is significantly stronger is off.
Ultimately, fire's legendaries will at best come even if the fight significantly favours them and otherwise fall short.
Having said that, I already mentioned fire in my post as one of the three casters that comes somewhat close when it comes to legendary reliance so even if you were to claim an equivalence here (which isn't supported by warcraftlogs by the way), that wouldn't change anything about the fact that the average spec benefits significantly less from legendaries than balance does.