All pilots should be aware that auto landing problems are not unique to helicopters too. It bites multicopters every now and then, like this one
And that fellow had the ground effect compensation enabled. The downfall of ArduPilot in this regard is that it uses GPS for horizontal guidance. All manned precision approach and landing systems for CAT II and CAT III use ground-based transmitters for vertical AND horizontal guidance. GPS is not trusted where human lives are at stake. Sure, you can fly a GPS RNAV approach but the minimums on that approach do not bring you down to wheels on the runway like a CAT III does.
Basically, ArduPilot is trying to make believe that we can accomplish CAT III results with non-precision RNAV horizontal guidance and it works most of the time. But it will eventually bite.