Sure, if people are using 3rd party sites that aren't advertised by the game itself, they are getting an unfair advantage over people who are playing the game as it is advertised. When people are storming about capping every gym in sight with 2800 Dragonites they found with Pokevision with great stats they sourced via Poke Advisor (disclaimer: never used it, if I'm wrong about how it works, fine, but if it shows you which of your dratini are going to have the best stats after the candy investment, thats a pretty big advantage over someone who can't see the hidden stats); then the whole gym battling part of the game is just a closed book to the "vanilla" player.
Like I said you're free to disagree, but until Niantic are saying "To get the most out of your pokemon, try visiting 3rd party sites such as:--" on their load in tips, then its an unfair advantage.
The whole argument is kinda moot, don't get me wrong But in a multiplayer competetive environment it shouldn't be about who datamines the servers for information the most.
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Do you curveball every throw. I was using up to 10 balls on a pokemon much more often than I'd like so worked out how to land curveballs and now its down to maybe three times max they'll pop back out of the ball. *Could be gamblers fallacy making me biased I was salty af about the "escaped 8+ times" situations.