It looks like the buffer to display output on the exiftool command processor is limited. After a certain number of lines, it stops displaying any output (but continues working until it's finished). In the attached example, I had selected ~400 files with most of them generating warnings.
[attachment deleted by admin]
There may be a limit to what the edit box in the dialog can take. Usually 32 KB of text.
Yep, I thought it was something like that.
I may post a feature request for a check if the window gets filled to display some sort of warning, like "WARNING: too many lines to display. You may not see the complete output of Exiftools".
At least, that would inform the user that it's not hanging.