What you're dealing with is basically just the fact that it's a slower payoff than a melee specialization.
Sticking with the cloaking and sniping tactics will payoff. You'll have to invest a lot into building your rifle up, but eventually most enemies will be one-shot kills unless they have multiple layers of shields.
Of course there is a lot more in the way of hybrid options in ME3 than there was in ME2 so some of it will be up to you and how you play the character.
Cloak + Shotgun = AMAZING!
Right now my load out is: Mantis V Wraith V Shuriken V
Cloak and Operational Mastery are at 6 and Fitness and Disruptor Ammo are at 5. Everything else is at 1 until I max the others out.