For any rural lines, If you do a good job of cutting all the trees near the powerlines it's much cheaper and minimizes customer down time. There really isn't anything else you can do. The cost of burying the lines per household is too high if you have to go 20 miles down a dirt road to farmer Fred. Like a million a household high.

It's difficult to trend, you aren't as susceptible to certain kinds of failure but it makes maintenance and repair much more difficult. Linemen can visually confirm hazards and sources in the air and change out broken or old equipment quickly. Underground requires a heavy amount of planning and engineering. I'm still a bigger fan of underground but it's not an easy swap.