RCC are not members, impossible to achieve full Christian unity.
Although the RC declaration (Unitatus Redintergratio) recognises that there are Christians outside of the WCC, they also declare that the RCC is the "true" church, contradicting it's absolutist nature.
Tensions within the WCC threaten it, for example some Protestant and Orthodox members consider 'missions' as 'inter-faith' dialogue and human rights rather than commission.