DatabaseBase::delete and DatabaseBase::update should always return a boolean, this is what the docs state and what appears to be happening when using MySQL. However, when using sqlite to run some of my tests (that run fine with MySQL), I'm getting some failures because I'm getting back a ResultWrapper object.
See failing test here: https://integration.mediawiki.org/ci/job/MediaWiki-Tests-Misc/1446/console
Version: 1.20.x
Severity: normal