Sabotage is a very tricky power that requires practice to use. All Geth are subseptable to Sabotage as well as the Cerberus Turrets and Atlas. Against things like troopers or Maraders, it causes a weapons overload effect when they try to fire their weapons which damages them and causes a stun similar to hitting them with overload.
What makes Sabotage tricky is that it's a burst effect similar to overload and may affect a target NEXT to the Geth you were attempting to hack. You can negate this by targeting something in the open.
You CANNOT sabotage cloaked Geth Hunters. You ether need to drop their shields or hit them when they drop the cloak.
Sabotage takes a few seconds to actually rewrite the code of the object so they may freeze in place a bit or continue to attack you before it takes hold. THIS is the reason so many players may get frustrated. What you need to watch for is glowing orange lines washing over the target. If you see these lines you've successfully hit the target and in a few seconds it will turn on it's foes if there are any in proximity. If no one is close, it will simply stand there and not fire on you. When the lines wear off, it will reset back to it's "Kill Players" mindset and resume the attack. The clock doesn't start ticking for wearing off until the power takes hold.
Sadly (and this is the source of my frustration) the glowy lines seem to attract the attention of other players and unless it's a Geth Pyro, Prime, or Atlas, it doesn't live very long enough to serve as a distraction.
As addressed by the others, it does have diminishing returns when used consecutively on the same target. By the 4th pop, you may as well be hitting it with blanks as it will have no effect. Switching up targets is advisable as most of the enemies in close proximity to that unit will ignore you unless you're in melee range to deal with their rogue ally.
And yes, you will get credit for killing enemies with/under a Sabotage.
DC4213 [Geth Prime] Geth Trooper
DC4213 [Atlas] Centurion
DC4213 [Turret] Combat Engineer