Potential hazards include:
Microplastics - if plastic is used in our design, then when the product is disposed of it may decompose into microplastics, worsening an already existing pollution problem.
Electronic waste - it is possible to recycle electronics or otherwise properly dispose of them, but many people do not. Like with microplastics this is already a problem for society and by using electronic parts in our design we risk contributing to it.
Data theft - if someone salvages parts from the device (or just steals the device), it could have the user's data still stored on it, leaking it.
Solutions include:
Designing the product so that either little plastic is used, or plastic components can be easily recycled (for example by making them detachable and out of recyclable plastics).
Providing a manual that explains how and where the product can be safely and effectively disposed of.
Encrypting user data on the device or including fail safes like wiping user data when the device gets plugged in for the first time.