Yea that would be pretty broken. Its super easy to find wild Dragonite so youd be able to catch a huge amount of them (or any other rare/ strong pokemon) and just "create" them for no candy investment. It is possible to find the legendary birds in the wild too. It could also be used to trade a pokemon caught in GO more than once. When you transfer to Lets Go and catch the pokemon there, the game acts if you caught it on Lets Go and replaces all of the Go data. Such as locations, CP, original trainer. etc.There for you transfer it back to GO and could potentially trade it again in Go? I'm sure they could figure a block for that though. Either way it takes the spirit of Go away and imo kinda of breaks it. Only for gen 1 at least.
GamePress
Would being able to transfer Pokemon from Let's Go be game-breaking?
Say if one day Niantic says this:
Attention all trainers,
"Professor Willow has finally discovered a way to transfer Pokemon from the world of Kanto to your device! Now any Pokemon from Gen 1 can be transported back and forth, with only a 12-24 hour penalty in between transfers."
Would this break the game in any way? Should Legendaries and Mew be banned from cross game transfer? Mewtwo only spawns once (at least from what I've heard) and the birds don't spawn much either.
