I like my Quarian Engineer, and while I like Infiltrator in single player, it just doesn't feel as powerful in multiplayer. Slowly one shotting shit is great, until you have waves of things charging at you.
I found the Human Infiltrator is actually extremely powerful as long as you utilize her right. Here is a few things I learned.
Never use stealth as the main form of attacking unless you focus on the melee and close range fighting. The fact is that once the enemy "loses" you they start looking for other enemies, which often means they move around more, thus making them harder to hit (much easier to hit an enemy storming towards then going off in a random direction.) However, if you want to be a close range fighter you are deadly, focusing on the ability to use powers while cloaks and get the melee bonus, taking a shotgun with you for those times you can't smack someone in the head.
If you do decide to stay range then forget going far into cloaking and instead max out Cryo Blast, focusing on the freeze duration, weakening, and recharge speed. It's amazing when a trooper runs at you or even tries to hide, so you blast them, forcing them to stand, fire a blast to kill them, fire another one to get the new troops coming around the corner, reload, fire again killing him, and repeat.
Also, you are basically the groups survival line. I can't tell you how many games I salvaged on my Infiltrator by simply saving my cloak for those times we get in a really bad spot. We get surrounded, my team is pretty much all dead, so I cloak and make a run for the other side of the map. Once over there, the enemies have to slowly figure out where I am and start coming towards me, at which point I am now recharged, refreshed on ammo, and waiting for clear headshots. If they converge on me I cloak again and run to another good sniper spot until they are all dead. You can't easily do this on other classes because trying to run out of the trap means you get pelted by everything in shooting distance and are often either weak or dead from it.
Have yet to unlock Quarian Engineer, but it seems pretty fun.