Iterative prototyping is great. The Cerny method is mostly useful for games that have tons of content, rather than systems. For example: an RPG that has tons of areas and weapons and characters. It's totally a good idea to first build a vertical slice with only one area, and a few characters and weapons...

But for systems-heavy games this falls apart. For a game like SimCity or Minecraft, making a proper vertical slice would mean building 70% of the game! That doesn't really help de-risk anything. :). For systems games it's much better to do iterative prototypes, accumulating systems with increasingly longer feedback loops...

(Source: my experience making systems-heavy games :) )