The editor is pretty much the same as Radiant, with a few notable differences (terrain inspector, new rotation widget, etc).
I would describe it as straightforward, rather than simple. There are still restraints on what you can make, they’re just different to what you know from mapping for other, smaller-scale FPS games.
It’s not quite that simple
Once you’ve made the terrain you still need to decide what to texture it with - the terrain inspector et al just let you decide how and where to apply those textures. Deciding where to use undergrowth textures for foliage, or worn routes for well-travelled areas is still a hands-on process.