I am frequently getting Wikibase Selenium failures on Vector. Usually these are due to race conditions and false positives. A recheck usually restores the problem but this wastes a lot of time in developer productivity and adds to CI traffic/slowdown.
As a consumer of these tests, there seems little value in running all browser tests on extensions and skins given we have a daily job that just does that. The only exception is for core changes, where I feel it is valuable to understand how a change there can impact extensions and skins.
Core: Run browser tests of all skins, core and extensions
Skins: Run browser tests of the skin.
Extensions: Run browser tests of the extension and all skins