The speed at which it renders, how easy you can interact with lighting etc., ability for multiple users in one project (very useful when working collaboratively). I have not found any real problems yet but if there is something please let me know! We are using luxrender as renderer instead traditional rasterization techniques such us Vray or mental ray/optix due its high performance rendering capabilities compared other free software like blender's v-Ray engine / 3dsmax render farm supportโฆ