Placate has worked 100% o the time for me, the only reason Im spotted is...
I'm hit during the animation
I did it on a Rikti Drone (NEVER!)
I used it in PvP on someone with high perception
all answered for...
in pvp even when you use it on someone with high perception, he might still see you but he is under the control of a status effect "placated". This kinda works as a reverse taunt. Instead of forcing the player to target you, you force the player to target something else. I cursed this alot when playing duke in pvp and I almost got a stalker down, only to get placated and just see the bugger run away while I can only chase him but not target him 
"god, how many devs did hami have to sleep with to get ED?"
Total Cat @Stagefright
trust me i just got plety of dept
also this just to similar level enemys 
trust me i just got plety of dept
also this to similar level enemys as well as higher lvl enemys
donno about anyone else but when ever i use placate i seem to have very bad accuracy anyone know why