Asking a question like "can you make an RPG with playmaker?" is kind of weird, because its not an engine like Unity. You can make an RPG with Unity, and you can use Playmaker to do certain things but designing your voxel based terrain system isn't one of those things. You need to code that, theres no way around it.
Playmaker gives you a visual representation of logic. The flow is managed by states, such as a trigger being in a state like "Player is not present" and then changing states to "Player is present".
In those states you stack up Actions, they're little mini scripts that you can combine together and do lots of different things. Almost anything a designer would need to do. Anything from interactive level things to triggering changes to any part of the game on the fly based on conditional variables all the way to designing an AI system. You can make your own custom actions by coding them and use them in the visual editor if you like. You could make your own scripted actions that are catered only to your game even, and they're just as usable as any other action in the graph view.
Now, if you had a voxel based terrain system from the asset store or something, then you could use Playmaker to access parts of that script and use them in some logic state. You can access pretty much any public variable in any script anywhere in the game and manipulate it if you want, actually.
So its very flexible. Much of the "its limited" train of thought seems to come from veteran coders that are capable of coding something excellent just as fast as you could design it visually with playmaker.
It's really great for just about anyone though, as designing things visually like AI has big benefits and doing it without any code is a big boon to things as a whole. Designers love it because they don't have to tell the coder or animator to make a script or small animation for their level since they can just do it themselves in Playmaker.
So basically its irrelevant what genre game you're making, its a tool that lets you build logic systems, events and states and interact with fundamental parts of the Unity Engine and scripts.
