If you insist on having six degrees of freedom, the wobble in your joints and slides will compound on itself and kill your repeatability. What to do? This beast from the university of Twente cheats - no bearings, no slides, it is all "flexures": bits that bend. The ball joints are really three nested flexures with the bend lines intersecting at the centers of the imaginary sphere. The bearings for the motors are also missing. The motors only turn 60 degrees and are mounted on butterfly flexures instead.
Maybe this will end up being used in semi-conductor manufacturing, where sub-micron repeatability matters.
There doesn't seem to be anything here yet