If your device was running iOS 7 or later, and you had set a passcode, your data would have been encrypted. So to tecover data from a wiped iPhone, the attacker would have to obtain the device, reconstruct the data from the hardware and additionally decrypt it by hacking your passcode or using a NAND flashing method (by creating a copy of your phone, testing passcodes and re-making a copy for every failed attempt).
It's not impossible, but highly improbable. You can however take many steps to make this much much harde for future thieves, for example by setting a more complex passcode.