mediawiki-selenium 1.0, the "Environment Abstraction Layer" allows browser tests to handle multiple users and multiple wikis, which is very difficult otherwise.
Beta Cluster stability - Able to test wide-impact changes (eg: changing the apache user to www-data) before applying to production
We're unable to properly run continuous integration on our extension, if it depends on Composer modules which are not already checked out in the mediawiki-core/vendor repo.
We really need support in Vagrant to enable a second instance of the mediawiki-core code, using the fundraising branch. The "multiwiki" role only diverges at the extension and configuration level. Not having this blocks us from getting any casual developers either from WMF or volunteers and keeps fr-tech in a silo as a result.