I think the maximum layer limitation of three is just not up to date! 
( I use a material with seven layers with diffuse, height, specular and normal map for each layer. )
My landscape uses seven generated maps from World Machine and on most places all maps are within one component.
And I want to use the layer maps to plant foliage automatically, so I just can’t use atlases.
Please support eight layers to paint the terrain any time soon…
Do you have a link to the Trello card where I can vote for more layers ?