I still keep meaning to play with kerythea, but my general thoughts on render engines I have tried -

Don't underestimate the internal renderer - especially once you get the hang on nodes, it works very well and can cope with high scale.

I've used indigo quite a bit it can cope with a fair bit of scale, but does hog memory, also as it's an unbiased renderer it's slow.

I've played a little with LuxRender a bit too, it's quite good so far, but I haven't thrown any large scale scenes at it.

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Toolset: Blender, GIMP, Indigo Render, LuxRender, TopMod, Knotplot, Ivy Gen, Plant Studio.