I just created my first rule yesterday! 
I too am concerned that there are gaps in the Actions implementation at the moment. According to this thread: Cannot Access Mapped SAML Properties inside Custom Action the only way to map custom SAML attributes into a user’s app_metadata is through a Rule. Losing this functionality will prevent our application from reading the roles sent by our customers’ IDPs.
Is the actions limitations page the only location to see the current gaps? Are there any plans to list gaps with expected migrations? Do you expect all gaps to be eliminated at EOL?