I came across a major blocker when dealing with upgrading some of the browser tests to async mode. In Two-Column-Edit-Conflict-Merge we're using waitForModuleState from the wdio-mediawiki's Util.js. In the browser tests of the Two-Column-Edit-Conflict-Merge extension we need to make sure that loading of the modules is finished before the test continues.
This did not work with the current way waitForModuleState method is implemented since it seems to be incompatible when using async mode.
As a workaround and proof of concept I added a minor fork of the Util.js to the extensions test suite, where I added async support. At least that fix should go upstream into a new wdio-mediawiki release.
See the patch where I got these insights from: https://gerrit.wikimedia.org/r/c/mediawiki/extensions/TwoColConflict/+/906082