CAUTION:

I would like to point that relates to an unfinished image. 

In any event, anything less than a 100% recovery will leave portions on 
the target image/drive that have not been written to by ddrescue. 

If copying to a brand new hard drive, those areas are (hopefully) likely 
to be zeros. But in any other case, those areas will contain whatever data 
was there previously. For someone that uses their system for data recovery 
on a regular basis, and is using image files or reusing hard drives, those 
areas could contain data from a previous recovery! This could be a privacy 
issue in some cases, but also could cause an issue with running any other 
sort of repair/file recovery tools on the recovered image/drive. 

The unrecovered parts could contain "garbage" data that could affect
accurate recovery. In these cases I would recommend using the fill mode 
of ddrescue to fill any unfinished/untried areas with zeros. 

Example command:

ddrescue --fill-mode=?/*- /dev/zero recovered_image logfile

This would write zeros to any portion of the recovery that was not
successfully read from the source.

Reference:
https://lists.gnu.org/archive/html/bug-ddrescue/2013-11/msg00011.html