Echo can target most hacked devices, but some devices — Augment Grafters, Augment Drills, all drones, Barrier Guards and OMNI Protectors, and the Security Hub and Mainframe Lock — are specifically exempt. If Echo has no valid targets, then it has no effect.
Echo is a relatively weak daemon; if it recaptures an important device like a Security Camera in a room you need to go through, then all you need to do is hack that device again. On the other hand, Echo often recaptures a device you don't need to hack:
When Echo is triggered by hacking a device, that device itself may be the one recaptured by Echo.