I think that is true, but with one caveat, you should test if the resetting actually worked.

I once had an Android phone with "full encryption" enabled that, after resetting, still had all the user data on it. It was obviously not only not reset, it wasn't even encrypted in the first place.

I am sure a lot of mobile phones have problems like that, because I think that smartphone manufacturers don't care about quality too much, as long the issues are not immediately apparent to an average user.

But if you can't recover the data afterwards yourself, I would agree that you can normally send it to recycling or repair.

Thank you. My comments might sound very strict and total. But of course everyone has different threats that they care about, and repair shops are probably not too sophisticated.

But that doesn't help you if the erase command was not implemented correctly.

And I am sure that you cannot implement an erase command from within the operating system. It has to be implemented in the flash memory controller. So if your hardware manufacturer got it wrong, you can't fix it.