I think hirelings can't get caught in your traps, similar to how your animals can't.
Other NPCs can, however. I've had a moronic NPC visitor around the homestead who first cut down a tree that was part of a trap array, and then proceeded to get caught in the trap that was no longer part of a trap array, and thus no longer visible to humans. The bugger insisted on everything being fine during the month or so he was stuck there due to injuries.
I've also found a dead trader in an isolated bear trap.
As indicated above, humans are able to detect trap fences and trap arrays, and thus avoid them, or systematically dismantle them (I've had the latter happen with a Njerp). Also note that humans have no problem climbing trap fences (although I wouldn't bet on hirelings following you over them: I haven't tried).
If you want to trap humans the traps should be isolated from each other and from trees, so you'd have to build a mine field of traps. It can be noted that traps seem to be less effective against critters who are actively attacking you, with Njerps only getting minor injuries and possibly being slowed down a little, and a wolf triggering a bear trap and then kept coming at my character (getting injured, but not stopped).