The Plasma Pistol is also a special-use gun, really. It's meant for stripping shields, which is does an amazing job at (if you're one to use it effectively). You're supposed to charge it, pop the shield off, then get a headshot with your other weapon. You should basically charge the Plasma Pistol, fire, pull out a Magnum, and get a one-shot kill.
The Needler, it's only problem is that it's got limited ammo. The needles track, and you can get 2 kills with a clip, if you're not wasteful with the ammo. Its weakness is the lack of ammo, but that's a good balance against the strength of the weapon itself. If the Needler had ammo like an Assault Rifle, and it was replaced by the Plasma Rifle in the Loadout screen, it'd be used heavily. They balanced its extreme strength with a short lifespan well.
Meanwhile, over in Black Ops, I'll call bull all day about your assessment of the guns. As someone who has the Diamond camo on 3 gun classes (Assault Rifle, Shotgun, Sniper Rifle), here's my assessment:
Assault Rifle: Most folks take the AN-94. The M8A1 gets used a LOT more than the SWAT-556, when comparing the burst guns. The SMR is used a LOT more than the FAL, when talking single-shots. I never really see anyone use the M27, and the MTAR isn't used much, either. I personally take the SCAR, as I think it's the best for my preferred style of distance shooting.
Shotguns: Really, the KSG is taken as a novelty, I think. The M1216 lacks the punch. The Remington is CLEARLY the popular pick here. Personally, I take the S12 for its fire rate (not a fan of the bursting of the M1216).
Sniper Rifles: NO ONE takes the Ballista. Really, it's split into two groups. Group One is the spray-and-pray bums who use the SVU-AS because it has no recoil. Group Two is the DSR-50 group, who are better shots, but the group is also filled with terrible quickscopers. I prefer the XPR-50, as it shoots faster than the DSR-50 (good when you have two opposing snipers in the window), while being stronger than the SVU-AS (good for the rare occasion someone gets behind me and I need to attempt a life-saving no-scope).
LMGs: I refuse to use these because they are a joke. The lack of recoil and the presence of Target Assist makes them too easy to use, and it's really frustrating. I'd say that the LSAT is BY FAR the most-used one of the group. Target Finder + LSAT = WAY overpowered.
SMGs: Another class I don't use much. I'm just not a spray-and-pray sprinter who lives off of the Target Assist. That said, the PDW-57 seems like the go-to of the class (though I personally hate it).
Pistols: KAP-40, bro. People use that more than any other pistol BY FAR. You'll get the rare Executioner or B23R user, but they're pretty uncommon. It's almost ALWAYS KAP-40.
And that's my point. You basically misstated two Halo guns as underpowered, when the real issue is that you don't seem to understand their use cases. However, there is a WHOLE CLASS (LMG) of overpowered guns in CoD. In the other classes, you have 1-2 contenders for most-popular, and that's whether the class has 4 guns or 7.
On top of that, CoD adds attachments where the balance is nonexistent. The Dual Band scope rules the snipers (I refuse to use it myself). The Target Finder is the clear-cut winner among scopes. LMGs are almost ALWAYS covered in Grip, FMJ, and a Dual Band of Target Finder. There's very little variation.
Really, if you want to talk about balance issues in Halo, it's the maps. They're WAY to asymmetrical, and it makes games like CtF unfair for one side (CtF on Abandon is the worst-balanced game of anything ever, regardless of genre). The only weapons with severe issues, in my opinion, are the Plasma Repeater and the Suppressor. The Assault Rifle just seems to outperform both among the automatic rifles. The Carbine's not all that great either, but it's usable.