To clarify a bit of the differences between FlowCanvas and NodeCanvas:
FlowCanvas provides a much more low-level visual scripting system and as such requires and as such required a better knowledge and understanding of the Unity API as well as programming (theory) in general. FlowCanvas is an event based system and as such is best suited for things that are event based in nature, such as level design events, UI or player input and interaction in general.
NodeCanvas on the other hand is a collection of systems (FSM, Behaviour Tree) which allow creating logic and AI based on the principles of those systems respectively. It includes pre-made Tasks (Action/Conditions) and does not require the same amount of programming understanding that FlowCanvas does. NodeCanvas is generally speaking best suited for AI although it can do more things.
So, NC and FC are not very different in what they can achieve but they do differ a lot in how those things are achieved, based on the architecture each graph system provides respectively (flowScripts, FSMs, BTs).
The integration between FlowCanvas and NodeCanvas, allows you to create NodeCanvas FSMs for example, where each state of that FSM, is a FlowCanvas flowscript to further create/model the behaviour of that state through FlowCanvas visual scripting graphs (flowScripts).
Please let me know if that answers your questions.