IMO/IME they are decently robust - I'd say atleast comparable with other head units - excepting a couple of points that aren't very hard to fix: the IDE cable (had the problem myself, recrimped the connectors in a small vice and no problems for 2 years now) and the IDE header (easily [if you have the proper equipment] solved by reflowing the connections). There's also one "hard" sensitive point - the fill nipple on the VFD. If dropped (don't do that!), depending on how it lands, there's a risk of the fill nipple breaking off, requiring the VFD to be replaced. I dropped a MkI from 1.5m height onto concrete and the nipple held, others have dropped theirs from lesser heights onto softer surfaces and had the nipple snap...

When I initially bought my first empeg there was a thread about a guy having it in a car that he drove pretty hard over "washboard" roads to get to surf spots - no problems while he never had had a CD player not skip on the same roads.
I've had mine for 3+ years in a lowered car on low profile tires (ie pretty hard ride) w/o any problems (excepting the IDE cable as stated above)

/Michael