@bobfandango that is an interesting question, and I will be curious to see if any other members weigh in on this.
I do not update FreeNAS at each release. Normally, I check the release notes and update if there is a fix for a problem I am concerned about or a new feature that I want. I also check the forums for comments people make about the versions - for instance, I skipped U4 because of the negative comments with that update. Over the last year, I went from 11.2RELEASE to 11.2U3 to U5 to U7. I did not have any problems.
I would not expect issues with an update from 11.2U3 to U7. Just be patient and let the process run to completion (sometimes updates take a while.) And I have three pieces of advice:
1) Back up your system configuration
2) Back up your system configuration
3) Back up your system configuration
With a saved copy of your system configuration, you can always rebuild with a new install without too much fuss.
Good luck.