We don't guarantee that and no one can.
Behavior will always differ and you always need to test on devices, however it should differ within reasonable sensible well documented parameters as long as you stick to our API's/guidelines.
Keep in mind that the device implements things the simulator can't e.g. native input (VKB), native widgets etc.
Networking is implemented natively in the devices, on iOS this is especially different as restrictions from Apple exist that do not exist in other platforms e.g.
https://www.codenameone.com/blog/ios-http-urls.html