Power-to-size ratio is just the tip of the iceberg; over half the rifles and almost all of the SMG in Mass Effect 3's multiplayer are 100% useless due to a combination of recoil, poor individual round damage, and poor effectiveness against most conventional targets.
The Saber/Paladin/Carnifex drum's been beaten quite a bit, but it bears repeating; there's nothing that the Sabre does that the Carnifex or Paladin, given a scope, doesn't do better for a fraction of the weight except sustainability - which, given our old friend the Clip Pack, isn't even that huge a downside, even if you're a complete yahoo who doesn't attempt resupplies during combat.
Hell, even the Viper has better conventional performance than the Saber, and I say that as a man who seriously likes him some Soldier class. The Saber just is not worth it, which is a damned shame for a weapon that's N7 rank.
With the exception of the Mattock, Revenant, and pre-nerf Falcon, the bulk of the rifles just aren't effective. Hell, the Revenant is a really iffy gun in and of itself; it kicks far too hard and its DPS far too fluctuating on any class that isn't a Turian - which is completely and utterly ridiculous. One might think that a professionally-trained Alliance soldier or a Krogan could handle that recoil without their aim attempting to emulate Parkinson's Disease, but you would be wrong.
The SMGs are even worse offenders than the rifles, being somehow LESS accurate when aimed.
I don't know who came up with this game's weapon damage and accuracy algorithms, but they're a mess. I've taken my Geth Pulse Rifle into matches and come out alive, but if it's taking me a clip or so of ammo to drop a single Silver Geth Trooper when I'm mostly scoring headshots, then I think it's safe to say that something is wrong.
To put it differently, I propose some simple logic puzzles:
1. Why would you take a weapon like the Tempest, Hurricane, Locust, or Shuriken, for example, when I can get better performance out of a decent-quality Heavy Pistol or a Shotgun, and they have a longer effective range as part of the bargain? The only thing the Tempest has over them is weight via Ultralight Materials - which is not enough to make it a favorable choice over a pistol or shotgun.
2. Why would you take a heavy weapon like the Saber or Revenant - a heavy weapon that fires single long-range slugs, and a heavy weapon with a high fire rate that is functionally only viable on one class - over weapons which accomplish much the same purpose for less weight? The Saber is simply not as efficient, effective, or damaging as the Viper Sniper Rifle, let alone a pimped-out Carnifex. The Revenant lays down a lot of firepower, but can really only be used effectively by one class; and even then, its spray is so wild that players may as well go for more efficient, accurate weapons with more reliable killing potential (I.E. the Mattock).
3. Why would anyone ever use a weapon like the Incisor, when everything it does is handled better by guns a fraction of its weight? The Raptor offers better damage and accuracy; the Viper offers better damage per-shot and better range; a quality heavy pistol with scope and barrel extension can flat-out out-perform it in every single catagory.
Unless these guns are made more viable, they simply won't be used, and the game will continue to revolve around Sniper Weapons, Pistols, and a few errant shotguns.
As an interesting aside, anyone with an N7 Eagle can get a glimpse of what a not-complete-fail automatic weapon is like. Seriously, that little bastard puts more effective DPS downrange than half of the Assault Rifle stable.