I had this issue, particularly with Poser Pro 2010.

However, with PP 2012 and Snarlygribbly's Scenefixer and EZSkin 2, fixing the most obviously problematic shaders is a matter of a few clicks. I almost never use vendors' materials as is, even when they say that they are optimized for Poser Pro 2012. But it doesn't take long to fix them sufficiently thanks to these tools.

If I use ready made lights, they will always require adjustment, which usually means deleting any that are not casting shadows and/or turning down the intensity. Most of my scenes are lit with no more than 3 lights plus IDL, unless there are a bunch of lamps or candles, in which case I use a point light for each with very low intensity and inverse square attenuation.

I've found adjusting hair materials to be the biggest PITA. And I've had to adjust the textures in Photoshop for many of them. But once it's done for each particular hair, I save the adjusted materials to the library and I don't have to repeat the process.

Most Poser renders look a bit flat to me, regardless of whether they are rendered with GC or without. Most photographs look flat as well. A quick curves adjustment in Photoshop, or similar adjustment in GIMP fixes it.

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PoserPro 2014, PS CS5.5 Ext, Nikon D300. Win 8, i7-4770 @ 3.4 GHz, AMD Radeon 8570, 12 GB RAM.