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 VWraith VShuriken 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.
gaming.stackexchange.com/users/20974/Steve the Maker
Focus on getting your ammo powers up and your Tactical Cloak as well.
Also make sure your putting points into fitness for the incressed health and shields.
Only melee if you are flanking and only if it gives you a good postion to take cover after you decloak.
gaming.stackexchange.com/users/21067/p3nquin
I would recommend trying to unlock the Quarian Infiltrator to gain access to Sabotage .
It's the equivalent of the AI Hacking ability in the previous games and there have been numerous threads popping up recently about how powerful the effect is since you can use it to turn Geth Prime and Geth Pyro enemies to your side temporarily.
You don't even need to deplete the enemy's shields before using it on them.
I believe that the fine gentlemen of the Beastie Boys said it best when they said Listen all of y'all
it's a sabotage
gaming.stackexchange.com/users/16793/Brysonic
I play my infiltrator as a techy.
Sabotage, cryo ammo, incinerate, specced to stack damage and do combos.
And using the two powers thing with cloak ability.
No need for a sniper just use the tempest 5.
High rate of fire freezes EVERYTHING with cryo ammo and allows for easy triple damage incinerate.
Sabotage also doubles this if it precedes them.
Then maxing out your gear for power speed and damage.
This hits like a truck, but i suggest palling around with EDI for her overload.
Very useful.
gaming.stackexchange.com/users/21401/Tyler
I play my Shepherd character as a distance infiltrator which is serving me well on Insanity.
I equip myself with the Widow with extended clip and damage, placing Disruptor ammo on to take out sheilded ememies.
My secondary weapon is the Paladin heavy pistol with Cryo ammo to freeze those unshilded enemies.
I use cloaking with the extended duration to get good sniper points and take out enemies from a far.
I find that Liara and either Janik or Garrus complement the build.
As well as having this build in the multiplayer I also like playing with a shotgun build.
Upgrade the melee bonuses as well as the ability to use a power when cloaked.
Then take a heavy hitting shotgun, of which I prefer the Claymore or Geth, then cloak, cryo then shotgun.
gaming.stackexchange.com/users/39360/SwankTribal11
I found this vídeo on youtube.
This guy are playing ME3 in insanity level without sniper... whit all points on melee dmg...
Watch 2:15
gaming.stackexchange.com/users/22880/CrazyHorse
In Mass Effect 3 I chose Infiltrator for the ability to snipe and Cloak, but so far I've found that I run out of Sinper Rifle ammo fairly quickly and  Cloak doesn't seem to help that much aside from the damage boost .  How can I play this class effectively and what skills should I focus on? The melee damage boost skill seems useless (except for the increased health) since even with cloak running in and bashing on enemies doesn't seem conducive to living, nor does it pair well with sniping.
How can I play Infiltrator effectively?
gaming.stackexchange.com
<mass-effect-3>
gaming.stackexchange.com/questions/53916
gaming.stackexchange.com/users/11920/Ben Brocka
You'll have to invest a lot in building your rifle up and getting your ammo powers up. Make sure you put points into fitness. Seems like there is no strict instructions as to how this class can be effective. It all comes down to taking advantage of mechanics and items.
Players are giving up their opinions regarding the effectiveness of the class.