The face rig for Kana in Sparkball was built with AdvancedSkeleton's face tool. That gets you a working joint layout and controls in an afternoon; what it does not get you is a face that holds up at the extremes. This page is about the pass that came after - the skinning and deformation work that turns a generated rig into one an animator can push.
An auto-rig solves placement and plumbing: joints in plausible positions, controls wired, a naming convention that other tools can read. On a stylised character it is the right starting point, and rebuilding that by hand would be work no one sees.
On Kana it did the bulk of the setup: the control GUI and the controls behind it. That is a real amount of work to hand-build, and none of it is what an animator judges the rig on, so it was kept as generated.
The eyes and the lips. A generated skin holds together across the middle of a slider range and comes apart at the ends, and the ends are exactly where an animator works - a shape is pushed to its limit precisely when it needs to read. Maxing the sliders on the eye and mouth shapes clipped geometry through itself, so the deformation around both was reweighted until the extremes stayed clean.
The rest of the face was left alone. The generated result was good enough everywhere it was not visibly failing, and rebuilding it would have been work that changes nothing on screen.
A full range-of-motion pass using AdvancedSkeleton's animation tool, which drives every control through its range so each shape can be watched at its limit rather than spot-checked by hand. Clipping shows up at the ends of the range, so the test has to reach them.