The new Fx node is pretty much the same as the Post Process Stack, but in node form. It is often a process of trial and error where am adjustment node would need to be replaced with another. Sometimes the node would need to be chained with another.
The Fx node makes that process easier by combining all the common adjustments inside a single node and allowing you to toggle them on and off. Additionally, it has its own Post Process Stack so you can apply further adjustments.
The documentation is being updated. Some sections may be incomplete.