Just my 2 cents, but all the applications that transfer the backups between Android and iOS do so by copying over the data locally, decrypting it completely, then copying it over to the new phone and encrypting it again with a custom version of WhatsApp and then replacing it with the App Store version.
In short, the software has access to all your chats in a completely decrypted form. Anyone offering the service locally is also going to have access to all your messages and media. By using such software or services, you are breaking the encryption and providing someone access to all your chats, so think about it twice before you do so.