Anytime you get error messages, the backup is suspect. The only way to tell would be to attempt to restore the files or see if you can look at them. I agree with GTK66 and always backup an image so I can restore my system if I need.
Is there anything specific about the files that were not backed up? Are they copyrighted protected or anything that might keep the permissions from allowing a backup?
Your backup was 86 G, much do you have on the partition you backed up? The sizes won't agree, of course, but it might give a hint if a problem exists.