I actually wouldn't say there's anything really overpowered in TS. Both sides just have their own powerful units (weather in speed, armor or firepower), which is exactly what balances the game, without making both sides prettymuch equal and thus dull.
Banshees are actually more useful for taking out some strong structures (like the Construction Yard, War Factory, or Tiberium Refinery), because it needs just as much runs (and the same number of Banshees as you would when using Orca Bombers) to take them out. This means the Banshees actually would have 1 run left, which indeed usually isn't enough to take anything out, but this does mean you could take a strong building out needing less Banshees (though using more runs) than you would need if you were using Orca Bombers.
The Nod APC is indeed a lot better than GDI's, but that's just what sets the sides apart. Where Nod would send in a bunch of Subterranean APCs filled with Engineers, Cyborgs (and Cyborg Commando) or/and Rocket Infantry, GDI would send in a couple Carryalls carrying a Mammoth Mk II, Disruptors and/or APCs filled with engineers, a Ghost Stalker or whatever.
I don't think the Disruptor is overpowered either, because for the Disruptor to damage, it really needs to get into the base first, which should be impossible without a Carryall if your opponent knows what he's doing (and has his base defended properly), unless you take out his defenses first. If you do send in the Disruptor with a Carryall, it runs great risk of being shot down and once it gets to the surface (seeming it's only 1 unit), it can be taken out easily if your opponent has his defenses or units at the right place; where nothing aside from an EMP Cannon (which can be avoided by sending in a couple empty APC's or Devil's Tongues as decoys first) would stop a Subterranean APC from getting to the base.
In short, for GDI frontal attacks are easier to preform and are sneaky attacks more risky.
For Nod sneaky attacks are easier to preform and are frontal attacks less effective.
Aside from this Nod also has better defense and can hide their entire base, but relies more on power, where GDI has a bit weaker defence, but won't suffer greatly from low power.
In the end it's all perfectly balanced if you ask me
Now about Firestorm... Juggernauts might indeed have more firepower than Nod's artillery, but that's only versus buildings. The juggernauts suck as defense and end up damaging your own units and structures.
The Cyborg Reaper appears to be the same; near friendly units or structures it ends up damaging them as well, but when attacking it has quite alot of firepower (more than the Titan and is also strong versus both vehicles and infantry, which makes up for it's higher price).