The Wrench programs are affected by Portcullis, so for example Wrench 4 breaks only 2 firewalls on a device with exactly 4 firewalls while Portcullis is active.
The Wrenches are very efficient in the PWR cost per firewall hacked, compared to other breaker programs without cooldowns. However, they can only be used on devices with exactly the right number of firewalls, so you will normally only want one in addition to a more versatile breaker.
A Wrench can still be used to hack devices with the wrong number of firewalls, so long as you can get it to the right number somehow. If a device has too many firewalls, another breaker can reduce it to the right number; to do this consistently, you will normally want Lockpick, Parasite or Data Blast to hack one firewall at a time. For devices with too few firewalls, you might wait for the firewalls to go up at alarm level 2 (or alarm level 3 in Beginner mode), or use Flare or Wildfire, or hack a device with Rubiks.