If a card could deal 5 damage to a target or give +1|+1 & momentum, it would cost more not equal to Lightning or Momentum. However the versatility tax is very low.
Well, true (thus why Purify's cost was increased). But usually this is such a small difference that it can be safely ignored, or addressed by a simple +1 cost increase, if the card really has multiple equally strong uses.
And as for all your guesses - Pulverizer is Rush-Break (it can destroy weapons too, but it usually is used to destroy shields and healing permanents), Eagle's Eye is Rush-Stall (all of its damage is blocked by shields), Arsenic is Rush-Break (poison sticks around, but it doesn't do anything to your opponent's damage), Blue Nymph is Stall-Break (high damage potential but really expensive and slow to get going). Fate Egg... doesn't count. 
I'll give you guys another hint - it's a creature. 
Pulverizer is Offense > Break >> Defense. It contains all 3 components though it is focused more towards offense and break.
What is the Defense aspect? It is able to remove almost any/all aggressive permanents like Nightfall Hourglass or Weapons.
Eagle's Eye is Defense > Offense > Break. It is not a perfect break card but neither is Scarab. Eagle's Eye like all weapons ignores CC. Bypassing/ignoring the most common defense is worth of being considered a break card.
Arsenic was not mentioned. Neither was Blue Nymph.
Your answer was Scarab | Elite Scarab lets examine this:
Offense:
2 attack / 2
+ 1 card | 3 attack / 2
+ 1 card + 1 upgrade (note usable devour only if 50% are played or if slowed down with a Pharoah.)
vs
3 attack / 2
+ 1 card | 5 attack / 2
+ 1 card + 1 upgrade
Conclusion: Can be transformed into a Rush card if supported with
synergy.
Defense:
Devour (note usable devour only if 50% are played or if slowed down with a Pharoah.)
Conclusion: Is a stall card if built around. Easy to use in this manner.
Break:
Vs
Removal: Many Scarabs (from a Pharaoh) will survive all but mass CC
Denial: Expensive to bring out but has a low per turn cost after the Pharaoh/Mummy
Negation: No special ability against most shields. With ample fodder it will overcome DR
Conclusion: Still needs a generator but it is mostly a Break cards
Summary:
Scarab is a Break > Stall >> Rush card. But it is not the only such card especially considering it uses a Pharaoh to be competent in those fields.
