The Primary Concern between Bane Lords and Wraith Lords is:

Gem Usage. I find it a much more efficent use of my gems to summon 3 BL's and equip them modestly that will outperform 1 WL with Full or even partial Equipment.

It may be that my playstyle does not limit me to my own dominion (meaning I don't build units to fight best in my own dominion) and the 'pure defense' unit that I use to clean my own Dominion late in the game (Where you'd use WL's) is the Demilich who, for what I use them for, is much more of a stopping unit with little to no equipment. It is also along a better research path in my mind.

That is just my opinion, but WL's can work if you need them to; just not how I use them.