I find that if they have were just about to make an attack and you placate them, they will continue the attack but after that they will de-aggro to you. Also Placate will only last a while, if the EB is programmed to attack you (i.e. if they spawn in an ambush) once placate wears off they will still go after you, even if you are out of sight. Also remember that some EB's have higher perception than regular baddies, I believe Luminary can see through stealth and just recently found out that Silver Mantis can as well, with them the first thing I do is placate them before they can attack then BU/AS them. Hope this helps 
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I've had no problems with placate .. infact it helped me solo lord recluse on the destiny arc even tho he had uber per and could spot me from a mile away.
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I've taken on Aurora Borealis and Luminary as EBs in the last few days and I've noticed a problem in that Placate is not breaking target lock for the EB. So I'd BU/AS/Placate/retreat out of sight and while I waited for everything to recharge they'd come looking for me, or attack me as soon as I showed my face for the next round. Most disconcerting.
Has anyone else seen this kind of thing since I7? It only seems to be EBs that do it (haven't tried any AVs though). Or has it, in fact, always been like this and I've been too wrapped up in myself to notice?
"If there's anything worse than being sacrificed, it's being sacrificed incompetantly."