It cannot be used to date volcanic rocks, for example.
To derive ages from such measurements, unprovable assumptions have to be made such as: There is plenty of evidence that the radioisotope dating systems are not the infallible techniques many think, and that they are not measuring millions of years. For example, deeper rocks often tend to give older “ages.” Creationists agree that the deeper rocks are generally older, but not by millions of years.
Geologist John Woodmorappe, in his devastating critique of radioactive dating, points out that there are other large-scale trends in the rocks that have nothing to do with radioactive decay.
It does not give dates of millions of years and when corrected properly fits well with the biblical flood.
There are various other radiometric dating methods used today to give ages of millions or billions of years for rocks.
Since the flood was accompanied by much volcanism (see Noah's Flood…, How did animals get from the Ark to isolated places? ), fossils formed in the early post-flood period would give radiocarbon ages older than they really are.
In summary, the carbon-14 method, when corrected for the effects of the flood, can give useful results, but needs to be applied carefully.
Clearly, such huge time periods cannot be fitted into the Bible without compromising what the Bible says about the goodness of God and the origin of sin, death and suffering—the reason Jesus came into the world (See Six Days? He said, This only makes sense with a time-line beginning with the creation week thousands of years ago.
It makes no sense at all if man appeared at the end of billions of years.
When a “date” differs from that expected, researchers readily invent excuses for rejecting the result.
The common application of such posterior reasoning shows that radiometric dating has serious problems.
This also has to be corrected for. Second, the ratio of C in the atmosphere at that time to be estimated, and so partial calibration of the “clock” is possible.