Q:

What are the consequences of "failing" a Guerrilla Ops mission by not killing all enemies?

While undertaking a Guerrilla Ops mission in XCOM 2, I found myself in a hopeless situation where I was heavily injured and massively outnumbered by enemies, but I had already recovered the package that I was there for.
I placed an evac zone and got everybody out, leaving all of the mission objectives complete except for the one to kill all aliens.
The score screen said "Mission failed" and rated the mission "Poor"; but the Council man congratulated me on retrieving the package, and the relevant Dark Event was averted.
So what did I lose out on by "failing" this mission?

A:

Guerrilla Ops have two rewards: countering a dark event (except for the very first guerrilla op, because there are no pending dark events when it takes place), and a reward that can be an engineer, a scientist, a soldier, supplies, or intel. Maybe something else too but those certainly are potential rewards.
It makes sense that by completing the timed objective the dark event gets countered anyway. After all that is the thematic reason you go to a guerrilla op. But you did not get the other reward, so that's what you missed out on by evacuating before eliminating opposition. And you also did not get the enemy corpses or any equipment left behind by your fallen soldiers not carried to the evac zone.

