Fault finding on overhead lines is much easier than locating a fault on underground cables,
I speak from experience having worked for electricity supply companies.
There's probably a reasonable amount of truth in that, given where most of the overhead lines are sited at the moment, but they don't seem to need too much time and effort to locate faults in cables in urban settings, for which underground is mostly the only reasonable and workable solution.
BT don't seem to have too much bother locating faults either, and their cables are almost all exclusively underground, in both ubran and rural settings.
BT though chose to lay their's adjacent to public roads in most cases, making access and maintenance/repair considerably quicker easier. Maybe if SSE adopted a similar model to BT for cabling, rather than their long-standing 'as the crow flies, regardless of what almost impassable terrain and/or extreme exposure that involves' routing policy, it would obviously considerably more expensive for the initial investment, but its difficult to see how they wouldn't be quids in in the long run in repair, maintenance and replacement cost savings. BT seem to think it works for them, otherwise they'd be reverting back to their old style poles and wires at every opportinity.
The current lifetime in Shetland of everything about the overhead power supply system seems to be a max of approx 40 years. Then it all needs to be dismantled, removed and a totally new one built from scratch. Underground cables would last much longer, and even when it came time to replace them, *if* they copied BT's system of laying piping in the trenches and then laying the cable itself inside that pipe aong with a draw rope, nothing needs to be replaced other than the cable itself, no digging or construction work of any kind is required. Just up to manholes, draw the new cable and another draw rope through with the existing draw rope, disconnect the old one, pull it out, connect up the new one, and head for home, job done,