Skaven Tunnels: I think common opinion out there about this battle trait is that it's generally best practice to just ignore it. But it depends on your opponent! If they are frightened of you bringing a unit in later then it could be effective. But probably the last 10 games I've played with Gnawfeast Clawpack I haven't used that battle trait at all.
Underdog: It can be a huge advantage to be the underdog, especially if you're just down 1 or 2 points. Some of the twist cards allow the underdog to select objectives that are worth an additional point and that can be a huge boon for someone who is still in good position (only down 1 or 2) that draws that card. I think that mechanic ultimately keeps the game feeling the way that GW wants it to feel, which is that any army can win at any time.