There existed an underdog system in original 'munda which worked perfectly fine in my opinion: The underdog received no in-game benefits but was awarded guaranteed XP based on the difference in gang rating and a possible income bonus if the underdog won, also based on difference in gang rating. This was great for two purposes: an underdog gang didn't have to win the battle, but could quickly advance to win the war (or at least later battles). If the game was one-sided, the underdog could bottle, do the post-game XP and leveling up to become more equiped to win the next time. I can lose 10 games in a row as long as my heavy weapon fighter receives +BS, my close combat fighters get +WS, some +T/W here and there. After those losses, my gang is starting to be real dangerous and would probably start winning instead.
Blood Bowl also has a great underdog system, but since that game is fixed length, the bonus is temporarily and applied before the game. This is done by calculating the difference in team rating, letting the underdog use the difference to purchase star players, special play cards and various other effects. Even if BB had a 'munda style system of awarding SSP (Star Player Points, same as XP in BB), you would have to suffer through 16 rounds (more or less) to receive those awards.
For Necromunda, I don't really need an underdog system that let's the underdog have a "fighting chance". I'm much more interested in the long term, and mainly just XP really. Give my underdog gang a XP bonus and I'll be happy to play any well equipped highly leveled up gang. Sure the bonus cards can be fun/helpful. Maybe a system like Blood Bowl where you could buy additional bonuses like Special Characters or Bounty Hunters to join you in the uphill battle. But mainly I'm interested in XP.
A few years back, before both Blood Bowl and Necromunda was re-launched, GW made Shadow War Armageddon. It was a mix of old Necromunda with many units and models from the 40k armies instead of gangs. What it severely lacked was any kind of underdog system (among other things).