After seeing a few extrusion-type 3D printers at Techshop (an "Up" and two MakerBots) produce inconsistent results and weak bonds between layers and strands, it strikes me that there's something fundamentally wrong here. This is a welding process. The extruded strand is being welded to the previous layer. Yet only one side of the weld is being heated. That produces bad welds in metal, and cold soby Nagle - Plastic Extruder Working Group
Both the Rostok and Stewart platform geometries are well-behaved geometries. There are many robots with 3-rail Rostok geometries. The ABB FlexPicker works that way, with three powered axes, and is not overconstrained. (The Adept Quattro has four powered axes, but the head orientation is fixed, so I think it is overconstrained.) It's a good mechanism for very fast positioning in a big workspace.by Nagle - Delta Machines