While attacking armies going strong on regen (like Necro or Aspects) in their lands, their retreat capability cost them near nothing. Since they heal ~40% (sometimes more) just in one turn, they can retreat and start the new turn fully healed.
The issue is the same with the Last Lords. Yeah, it cost dust, but very little to fully heal an halved HP army.
I'd suggest to not allow units to heal the turn they retreat