Decoy is super useful on any difficulty. It can be a bit tricky to use, but it is possible to lock down banshees, brutes and phantoms with it (probably any enemy really). I don't think you even need to max it up for it to be useful.I just unlocked the Salarian Engineer. What is a good build? Is decoy useful on silver/gold? I have 63 points to spend, so I can pretty much do any build right away.
I don't know any exact builds though. I'm levelling up my salarian engineer right now and I'm not completely sure where to put my points. I might use a reset if I completely mess up my point allocations.