You cannot apply shackles to enemies with phys armour.
That's kind of what I feared.
You also have to add to the equasion that killing an enemy gives you 2 AP, shackling him takes 1 with almost no return.
Also I feel like if enemies die while it is your turn even tho it says +2 AP you don't actually get them when it's your turn.
This basically reduces the usage of shackles to one specific scenario:
One enemy still has hp but no phys armor and on top of that you're surrounded by enemies and want to stop them attacking you.
Late game this won't happen too often, but I wonder how good this is in Act 1 with a purely wepon based build that uses shackles as utility.