First off I want to preface this post by saying that I think both factions are very capable in capable hands, and that any strengths and weaknesses are ultimately leveled out with player experience. Also there's a tl;dr at the bottom.
As it stands right now the vehicle balance is pretty ridiculous, and I believe the problem is how easy it is to do well with GDI vehicles. Meds and Mammoths have great firepower, and Meds and Orcas have great mobility. MRLS's have lock-on capabilities that guarantee hits outside of cover while Artillery needs to account for the distance and speed of their opponents. The result is, with the exception of the Mammoth tank's sluggishness, ALL GDI vehicles are move, point, and click to win. And this fact alone does lead to wins.
Now before you jump to conclusions, no, I'm not saying Nod can't put up a fight. I'm saying Nod puts up a fight less often, because the effective use of their vehicles requires more teamwork and more specific circumstances.
A perfect example of this is the stealth tank, as it's not even really a tank, and more akin to a stealth transport with damage capabilities. Pop out of stealth to attack ANYTHING and your stealth tank is put at a huge risk. Mammoths and Med tanks can pop you like a balloon and even infantry can put up a good fight until you shift-drive away from them, since you can't run anyone over when you've been detected, and firing your missiles is like rolling a pair of dice. As a stealth tank you are not something to be feared, you're a mosquito to be swatted away. Now put that same stank in the middle of a well-executed stank rush, and GDI is probably looking at a new dead building on their hands. How? What changed? Nothing, Nod just used teamwork and surprised the enemy. But when a stank attacks you on your own, isn't it still surprising? Now why is the $900 tank struggling to do anything?
It's the same situation for most of Nod's vehicles. Light tanks just don't get anything done. They don't have the damage, and their speed and low profile design is still easily shelled by tanks and locked on to by MRLS's and Mammoths. Light tanks are also all around outclassed by Med tanks, in 1:1 situations.
Apaches can't duck out of sniper fire easily, so their firepower is often nullified. They also have to deal with MRLS's firing at them from miles away, requiring them again to duck into cover. Same goes for Mammoths with rocket pods. Orcas are quick and responsive, and can more easily deal with sniper fire. But they still have to deal with missil- oh wait, no, NONE of Nod's ground vehicles can respond to Orca harrassment. Stanks can, but we've already been over how weak stanks are on their own, and their range is very short. Orcas are fast, they can fire and then outrange the stank's missiles if they need to.
Artillery is Orca food.
Also, Artillery in beta 4 will see even slower Artillery shells, so you'll need to get even better at adjusting for distance and leading your shots.
So, we have stealth tanks, light tanks, Apaches, and Artillery, all with glaring weaknesses. More weaknesses than GDI vehicles have that fulfill the same role. Now, what is the Med tank's weakness? Can you even think of one? It can kill light tanks. It can kill Artillery. It can kill stealth tanks. It can kill flame tanks. It's perfectly capable of handling infantry. And every other GDI vehicle can keep Apaches away from it.
So here's my point. GDI's armor is flat-out too good and too well-rounded. No, this isn't because it's just GDI's thing, how they're depicted in the C&C games. It's because your average joe Ren X player can pop into a tank or even an Orca and he won't run into any costly problems. That same guy in any of Nod's vehicles will find himself short one vehicle when he inevitably runs into the wrong situation. Multiply that difference over the course of a game, and you've got a serious problem.
tl;dr GDI stumbles into good teamwork and synergy by virtue of their design, and Nod takes the underdog thing too literally. It makes for frustrating games.