You can create any number of fields which are relations to the User type, or to a group type. So you can create a field for each of the RACI elements.
With automations, you can determine how/when these users/groups should be notified when the deliverable changes.
The Assignments extension is basically a many-to-many relation to the User type, but on steroids.
It has the extra benefits of providing automatic notifications when a user is assigned, or if a comment is added to an entity. So you might choose to use Assignments in place of one of the RACI elements.
However, it has the disadvantage of not being able to be renamed
Also you could manually add appropriate notifications for other user relations using automation rules if necessary (but just not for comments unfortunately).