Is it easy to add game-mechanics?

e.g., Class types, Inventory, Crafting?

Did you see the tutorials? There are examples for inventory.

What you mean by "Class types"?

I think he's building a fantasy-based game.

The term "easy" is a subjective term. If you mean it's a point-n-click approach to being able to add avatar classes, inventory or create a crafting system, then the answer is 'no'. You will have to code all these things like we are now doing.