Work = achieving what movements set out to do.
For example nonviolent action was effective to bring peace in the: Balkan conflict
and Ghandi used it to end British rule in India.
It helped bring about the Rose revolution in Georgia and regime change in a number of other places.
i would imagine success for XR would be for the UK government to commit to net zero targets aligned with what science, says is needed to stop catastrophic warming. Policies that protect and restore biodiversity. Citizen’s assemblies - a more participation heavy form of democracy than is practised currently. All sounding more achievable than overthrowing governments!