Hi
If you want to know what you can do then stick to allowableActions.
If you need to change access rights then you need to consider a bit more. CMIS ACLs may not give you the control you want (you may need more than Read, Write and All). If you delve into repository specific stuff your code will no longer be portable.
In Alfrecso, the CMIS API reports the best it can for ACLs, however ACLs are always reported as "inexact", meaning more can affect the decision. Alfresco can assign system wide rights, rights to owners, etc, that will not be reported. There is no idea of owner in CMIS. So allowableActions will always report what you can do. The ACL API only allows you to see/change node specific settings.
Hope this helps
Andy