When you crouch peek, normally your crouch+strafe and then crouch +strafe back. The plugin stops the double crouch motion so basically, you're stopped on your tracks while trying to crouch+strafe back.
On the nuke vents, you double crouch really quick to go in vents quickly (for instance when rushing inside through the squeaky door and want to plant lower. Well, since after the second crouch you should still keep the forward motion to actually go in the vent, with the plugin you're stop and can't actually go in the vent.
This is why I suggest allowing 2 ducks and stopping the 3rd one. In any case, allow a setting to determine how many ducks you'd like to allow in your server(s).