what happens if I don't write back the metadata on some files?

[neal]

What happens if I don't write back the metadata on some files?
I have 500+ files, mostly ai, eps, and a few jpg. Write back does not work on them.
Neal

[Mario]

Write back does not work on them.

What does this mean? Do you get an error? ExifTool reporting problems?
AI and EPS use XMP sidecar files. JPG use embedded XMP data.
I would no expect any problems when ExifTool writes to the XMP sidecars or the JPG.