Just putting out my experience in upgrading to v12 from 11.0.3.
Using the silent upgrade the upgrade got as far the Post stage where the upgrade kept falling over with a DB error. For some reason the details of the error are written in the main Sugar log and not in the upgrade log.
The errors turned out to be many date values of 0000-00-00 00:00:00. After fixing one group of errors I ran silent upgrade again with the "-S post switch" fixed the next group of errors and so on.
Once all zero values had been replaced the upgrade went smoothly.
Good luck.
