Well to someone as old in Necromunda terms as myself, what is in Necromunda: Underhive (the LRB) scarcely covers the whole game imo. It is nothing without the old Outlanders Expansion, which includes:
The Outlaw Rules
Ratskin Renegades
The Redemption
Scavvies
Spyrers
Wyrd Hired Guns
Pit Slave Hired Guns
A Bestiary of some form - Nik Jacos' 3 Parter (Can You Hear That?) is better than the old Bestiary in my opinion, shame it doesn't have all the Xenos though.
All the above imo are vital inclusions into the Game, and after a while you and your gaming buddies may want to expand into the Ash Wastes, or include even more madness in your games.
There are a few things you can never get enough of, and those are Scenarios, extra Hired Gun types and extra oddball stuff like Vehicles and other oddities to add to your games.
The scope is only as huge as what you and your gaming buddies want to add to it. The more you'll play the more you'll encounter stuff you'd like to add. Most of the extras are useful, and even if they are a little unbalanced, it adds flavour to the game. The best way to see if they are right for you is to try them. If you like them you use them again, it's as simple as that.