From a servicing standpoint, overhead lines are far easier to observe, inspect and repair than underground lines.  I discussed this and other pros and cons with a line engineer with SCE.  According to the engineer, his crews prefer servicing and troubleshooting overhead lines because the entire portion is visible.  If there are issues with an undergrounded circuit between transformers or other equipment, they pretty much have to replace the entire segment.  

Longer term, underground equipment and lines do have usable lifespans.  Which means there will come a point in the future where these lines WILL have to be replaced.  How long that lifespan is is dependent on the quality of the equipment and wires as well as how much load those circuits handle.  According to the engineer, early undergrounding equipment lifespans were approximately 30 years, give or take.  Most of what goes underground today will last 50 years or so, provided the electrical load remains at the rated capacity. 50 years is a pretty good length of time but as we know simply putting off a problem (the eventual need to replace the lines) into the future does not mean it will be addressed (or could be addressed) when that time comes. Repair and or replacement of undergrounded lines will almost certainly be more difficult to utility companies than the original undergrounding was (mostly performed by the developers).  

And that assumes that the raw material/specialized equipment is readily available and affordable.  

On a personal note, I lived in a subdivision (in Virginia) that was undergrounded in the early 60's with lines with a 30 year lifespan.  As expected by the late 80's the system was showing its age.  We would lose power every time it would rain more than one inch.  The utility could never locate the problematic section so after a year or so of undependable service they replaced the entire distribution section.

Makes you wonder which method is better in the long term.