Jim - I would say we do lose quite a percentage of flights due to not seeing the rig, compared to virtually none from having to go-around from an ILS. Although they both have a nominal minimum height of 200' in the case of the ILS, one only has to see the lights looking mostly downwards through 200' of cloud. Lets say the slant range is 300'. So even with cloud on the surface, provided its not too thick you will still get in. For offshore, at 200' you have to look through 3/4 mile of cloud - more than 10 times as much - so if you are in the bottom of the slighest bit of cloud, you won't see the installation.
SAS - limits on N Sea have been, for nearly as long as I have been flying on it, offset at 1.5 miles then continue to 0.75 miles range from the nearest bit of structure (front of radar blip). I think it used to be 0.5 miles in the late 70s but was never 0.25
AFAIK.
Although the technology certainly exists (and is installed in the EC225 amongst others) to effectively auto-hover-taxi from MAP to destination, in practice one can get very disorientated and of course the failure of the system has to be taken into account so that the pilot can recover safely. In the case of auto-hover over the ocean for SAR, you can just go ahead and up which is not too bad, but if there is a large chunk of metal just ahead it all gets rather more tricky.