AC loses due to reactive impedance and skin effect should always be higher than DC transmission. DC might make sense if the generation is solar based, as it naturally generates DC, although converting it to higher voltages must have an AC step.
I think it's only efficient for the largest capacity lines tho
If I remember correctly that actually makes things much worse if you've operated at one potential for a considerable amount of time.
You could operate the cable at AC ~ 0.1Hz but your ability to actually transmit MWH would suck.
Whatever, the ground currents penetrate very deep into the earths crust thus equally effecting buried cables.
Its the difference between just shifting the frequency or phase on your local control clock at the substation, and trying to coordinate with another state/territory/nation's entire AC generation infrastructure to keep everything in phase.