It cannot be used to date volcanic rocks, for example.The rate of decay of N in 5,730 years (plus or minus 40 years).These techniques are applied to igneous rocks, and are normally seen as giving the time since solidification.
There are various other radiometric dating methods used today to give ages of millions or billions of years for rocks.
Also, the Genesis flood would have greatly upset the carbon balance.
The flood buried a huge amount of carbon, which became coal, oil, etc., lowering the total C ratio in plants/animals/the atmosphere before the flood had to be lower than what it is now.
The strength of the Earth's magnetic field affects the amount of cosmic rays entering the atmosphere.
A stronger magnetic field deflects more cosmic rays away from the Earth.