RepRap/Mendal/Prnrbot/Makerbot all use a plastic extrusion mechansim (think robo-spider) which can make a lot of cool things but cannot make a bunch of other things. In particular 'spinning' machines (as I tend to think of these) cannot make large overhangs because the material they are using has no structural stability while being extruded. [1] The "Dual 'struder" on the Makerbot Replicator opens the possibility of a 'support' material that can be removed, this requires filling in with the support material to support the non-support material being extruded over it. The support material is later removed with a solvent.

In powder based machines the 'unbound' powder it its own support. So overhangs are not an issue. The challenge with powder machines has always been that the materials have relatively poor structural stability (in the cornstarch variety) or require a precise (and) delicate sintering process, or a 'firing' type process. In all cases having both bound and unbound material present can challenge the finishing stages.

[1] These machines melt ABS plastic to the weak/sticky stage as they push it out (typically 220 - 240 degrees). At that temperature a strand cannot hold its own weight.