The devices in question all were fat formatted, for which it is quite easy to reconstruct most of the "deleted" content using readily available simple tools (which I did).
But that was some time in the past, nowadays most systems use some kind of base encryption, making it less likely to easily find something.
The phone, on the other hand, was just not doing a factory reset before selling it, but only deleting files (and missing stuff, like some mail folders) and not having a password-protection at all... 😬