Though not common, one of the more frequent...
?
If you're going to power from a local AC source it's probably going to be cheaper and easier to just power the camera with AC or DC and forget the PoE injector (another component to fail).
It goes without saying that if you can plug in the camera directly to local power then you should do so.
The dual-power setup with PoE and DC provides more true resiliency, though it can make it harder to do any kind of full power cycle if needed.
It goes almost without saying that if you can plug in the camera directly to local power then you should do so.
If the camera is PoE only, then the local PoE injector is probably slightly more reliable,
Exponentially more reliable in the 'failure modes' cited by the OP, 'if the LAN cable is disconnected or cut', because the LAN cable need only be 6 inches long, and the injector can be put in a junction box behind the camera.
but significantly more expensive, than a PoE switch option
if you can plug in the camera directly to local power then you should do so.