On paper the Arc Pistol should be an amazing weapon: its normal fire is as fast and powerful as the Predator, making it ideal for CQC against low-level mobs, and yet it has a charged shot that can deal magnum-level damage for use against armored and high-HP targets (It is possible to perform one-shot headshot kills with a charged Arc Pistol). And the Arc Pistol is balanced to take account of the latter fact, thus it is one of the heavier of the pistols as a result.
In practice however the Arc Pistol is gimped because in order to release magnum-level damage, it has to be charged. Unfortunately, charging kills the Arc Pistol's rate of fire significantly, and both MP and SP gameplay show that the most effective weapons have a good combination of RoF and attack damage. The Widow and Claymore have slow RoF but compensate by dealing incredible damage that can kill opponents in a single shot, and the Geth Plasma Shotgun and Graal Spike Thrower deal incredible damage when charged. The charged Arc Pistol only deals damage similar to a Carnifex, and yet the severe reduction to RoF means that it isn't as powerful as the Carnifex in practical combat since the latter weapon has a faster RoF - and the Carnifex already 2-3 bodyshots to kill even Husks. And since the Arc Pistol's standard attack is middling at best, that means that the Arc Pistol has all the weaknesses of the "magnum" Heavy Pistols with none of the advantages. At this point it's more effective to simply stick to the standard attack for the RoF, but if you're going for pure RoF the Predator or Phalanx would be more preferable choices due to being much lighter. Along with the fact that its weight makes it difficult to use for power users and you have a Heavy Pistol that doesn't really have much to offer on the field.
The only way to make the Arc Pistol viable is either to decrease its encumbrance (i.e. make it lighter), or drastically increase the damage of the Charged Shot - say, +40 to +50% more damage - to compensate for the RoF decrease and make it viable as a legitimate combat option.