I've found the app to be hard on hard. But I've only gone through 1 "hard" campaign. The problem I have with hard is that the game takes forever to actually finish with a loss. So I'd advise you to surrender when you're sure you can't come back. I mean - when you're sure sure sure.
Other than that, it's a blast (from a blaster pointed your way )
I think he's referring to the time between "knowing you will lose" to "actually losing" and recommends just surrendering the mission once you reach the former instead of dragging it out to conclusion as both are counted as the same loss.
You would be missing out on reputation (which benefits you) if you surrender so I'd recommend on playing fast to completion with a new goal of killing as much stuff as possible.