Personally I wouldn't worry about it. While there are ways of removing the flash stoarage from one motherboard and soldering it onto another working one, this process is extremely difficult. In other words someone would have to have very strong motivations for doing so, as the proccess is very involved. However even if someone had a reason and manged to do this they would still be greeted with a passowrd screen once they got the phone working. If someone tried to then restore the phone (deleting all the data) they would also be met with an iCloud activation lock preventing them from setting it up without your apple ID or iPhone passcode (assuming you had find my iPhone turned on).
If you're still worried about your data you can also remotely erase the data for that device in the find my app, meaning if someone hypothetically managed to go through all that, the phone would be reset as soon as it connected to a network.
I'm not entirely sure how Apple's recycling system works but I would assume if the phone is seperated up into companents and then materials that some components such as the flash storage might be destroyed or seperated to such a degree they become disfunctional. I found this thread on the topic: https://discussions.apple.com/thread/5100096
All in all unless you're keeping nuclear launch codes or something on that phone I wouldn't be too worried about it if you're recycling it with Apple.