This article will tell you a little about CO2 and the problems that can happen when using it. While it is inferior to compressed air, it is more affordable. Compressed air is much more dependable.
CO2 works fine for many paintballers, and thus they don't opt for the more expensive power source, HPA. While HPA is more consistent it is also more expensive
In cold weather CO2 can be lots of trouble. Problems such as inconsistency, sputtering, choking, shooting snow, gun getting cold, tank getting cold, and your gun "freezing up" are what you can experience while using it.
You probably will experience these problems during periods of rapid shooting. When you are rapidly firing it is usually during a time when it is extremely important that you get lots of paintballs in the air. It is during times like these that you especially don't want any problems to arise.
In a tank the CO2 is in both gas form and liquid form. The gas is what you want your gun to use, while the liquid is what you want to keep out. When you shoot the gas, more liquid will turn into gas. When you are shooting rapidly the liquid often isn't able to turn into gas quickly enough, so it goes into your paintball marker.
Liquid entering your gun can cause horrible consistency from shot to shot. One shot may be fine, but then you may notice that the shots after it all travel shorter.
As you can see, when you are trying to hit a target, inconsistent velocity can make it extremely difficult to ht a target. There are times when you need to shoot fast, and when doing so, it makes it difficult to effectively shoot fast when each shot goes shorter.
So what can you use that won't give you problems with consistency? Compressed air (also called HPA) is a superior alternative. It works much better in paintball guns, is more consistent and isn't hindered by cold weather.