Easy explanation for this: they're always sandbagging because the alternative is Uriel getting awfully smitey. As far as we can tell, the White God and his minions may only act to counter magical attacks upon human agency, and only perfectly in kind. Six words of despair become six words of informative hope. The mass use of Hellfire gets Harry granted the ability to use Soulfire. The White Council trying to unilaterally kill Molly becomes Michael singlehandedly saving multiple members of the High Council, Showing Up, clearly On the Clock for the Almighty, wielding the Sword of Love. We can be fairly certain that, if Nicky and the Nickelheads started going on a superpowered rampage, not only would every Sword immediately have a bearer, but all of them would begin channeling absolutely astonishing amounts of power and making them completely unstoppable. Think Murphy charging through thousands of vampires and absolutely curbstomping all of them, taken up to eleven, but with only a maximum of thirty targets.
So, instead of using their supernatural prowess at every available opportunity, they work mostly in the shadows. Nicodemus has been building up influence, wealth, power, and a cult following for literally thousands of years because the angels can't do a damn thing to stop the mundane humans he employs from making those decisions. They only go full monster mode against people who can magically defend themselves, like Harry, or against the Knights of the Cross. They have learned over a very, very long time that using anything but their immortality and intelligence to accomplish their goals tends to go poorly for them.
