there are two scenarios i'm thinking of:
(1) grails.gorm.failOnError = true
can't really simulate this in unit test, so basically requires consistent (and redundant) <domain instance>.save(failOnError: true) to make unit tests consistent with app at runtime, which eliminates the benefit of that setting.
see this thread for reference: http://grails.1312388.n4.nabble.com/unit-testing-grails-gorm-failOnError-true-td4231435.html
sometimes unit tests fail and sometimes log output can be a huge benefit in tracking down the issue.
the now defunct 'mockLogging(class, enableDebug=true)' attempted to address this, but is no longer in play in 2.x+
see this thread for reference: http://grails.1312388.n4.nabble.com/grails-2-0-0-RC1-mockLogging-td3993095.html
on one level w.r.t. the 'dry' principal it would be cool to have unit tests 'honor' config.groovy, but i'm sure it's not that simple, so i'll leave solutioning to the experts