Below is a couple of sample pages from Radiometric Dating Methods .
You are looking at two pages of the description of one of the techniques. In the description, there are warnings given about when the technique is applicable. There is always more than one technqiue applied to any rock sample in order to cross check the validity of the results.
For example, a study of the Amitsoq gneisses from western Greenland used five different radiometric dating methods to examine twelve samples and achieved agreement to within 30 Ma on an age of 3,640 Ma.[Dalrymple, G.B.1991. The Age of the Earth. Stanford University Press. ISBN 9780804715690]
Any one method is quite complex. These are not dumb people making stuff up. They have rigorous, peer reviewed and tested research underlying their techniques.