When approaching a device, you may want to consider the following:
Some manufacturers ship their devices with fully or partially unlocked bootloaders. Imaging such devices usually presents no problem (unless data is encrypted).
Some manufacturers ship their devices with locked bootloaders, officially allowing users to unlock them (for example, Samsung, Motorola, SONY, Nexus devices, and many others). If the user unlocked the bootloader, there is a high probability of successful imaging (unless an anti-forensic technique was employed).
In many cases, manufacturers and/or carriers restrict bootloader unlocks. In some cases, it is possible to bypass these restrictions in some models (the chipset and manufacturer of the device are important). Some chipset makers and device OEMs have backdoors (used for service-level access), while some can be exploited to bypass bootloader lock and boot into a custom recovery (which does not necessarily mean that you can permanently...