And I have used the Mattock in ME2, great weapon on lower difficulties, but say you play a Soldier on Insanity as I have, its practically worthless. The gun carries enough ammo for two three enemies because of how strong their protection is. When you have to hang back and take enemies out afar in a long firefight, it doesn't keep up to the Revanent.
I found when your another class that has assualt rifle training, the Mattock is a good choice because you have powers for backup when low on ammo, but when your a soldier and only have guns, its quite difficult.
(Played the game through multiple insanity runthroughs)
Mattock is king when used in Adrenaline Rush. It drops most enemies in a few shots if I remember correctly. Add to the fact it's ridiculously easy to get headshot after headshot in the time dilation (even without it?). And you should have AR up every few seconds. I seldom had a reason to fire out of AR as a soldier. Mattock in Adrenaline Rush > Revvy in Adrenaline Rush.
Oh and that's probably why I took reave as a bonus power. Just in case. But you can pick up enough ammo to keep the Mattock viable in game most of the time.
It's all my opinion. But just love the Mattock for precision tearing down of enemies. Revvy I used more for a run and gun beefy soldier playthrough
Off topic. The "tracking" capability of the GPS was pretty useful at range despite it's slow projectile (?) speed. Just have to anticipate the enemy a bit the farther away you are. Particularly useful (as a vanguard) for those situations where getting in close/charging in would mean instant death (again on insanity)