I have had about 5 service change outs that had the old panel on wood backboards. All had rotted wood. Have yet to have masonary/ concrete rot out. Not all the block walls were as good as new but have not had any trouble mounting the new panel on the walls. Usually use lead set type anchors to mount the panel Sometimes am able to re use those type of anchors if the screws have not rusted.
Of course differenr areas have different work practices depending on local factors.
Almost all resi panels are in the house here only the meter socket is outside.
