I find it is useless on the slow machines - tank and harvester. Results on medium speed machines (hunters) are mixed. I have caused them to fight each other but haven’t yet got a kill. Results on runner packs are really good because runners react fast so you get a lot of your 40 seconds worth of hack time spent in combat.
The best targets are mixed groups so runners escorting a harvester, or one or two hunters among runners. Hack a runner and watch the fun.
I hit a pack of 4 runners guarding a beacon yesterday - the rogue one killed one of its mates then the two others turned on the hacked one and killed it. Meanwhile I sniped one of the survivors leaving only one to rush me who I took down easily as it ran towards me. Two sniper shots to kill four runners.
My second good result was 2 runners and a hunter guarding the Hagaboda radio mast in the Loud and Clear mission. I hacked one of the runners which killed its buddy, then the hunter killed it leaving me just the hunter to deal with.
My best result was a few minutes later when I ran over to the two police cars to get the radio shack key. Several runners and another hunter came at me so I ran away and hid in the trees. The machines stopped chasing me and milled around the two police cars so I hacked a runner and all hell broke loose with one of the machines hitting a police car in the firefight and both exploded taking out the entire group of 6 doggies and a hunter in one big bang.
It is annoying though when the hack fails and a pack of angry hunters come after you. You need good running shoes half the times you use the hacker skill. On the rare occasions it works well, its fun watching the machines having a bar room brawl.
6 dead runners and a hunter near the burning cop cars. It was a good thing I got the key out of the cars first.