Here we go again...
Cearbhall, no-one has said that U-Pb dating (or any kind of geochemical dating) is one hundred percent accurate. It is based, as all scientific
theories are, on assumptions and inferences, some of which - *gasp* - may be incorrect.
But we act to minimise the chances of a flawed result. Samples used for U-Pb dating are selected carefully, ensuring that no sign of erosion (and
therefore no possible contaminant) are present. Samples are taken from a geographical and chronological range, examined, dated and compared, and then
a picture is formed of "contextualised" samples.
Even at this stage, U-Pb dating isn't relied on by itself, but is checked and compared with other dating methods, geological evidence, and every
other investigative technique we can think of. Only then do scientists put forward an unconfirmed date for the area they are studying.
It's not guesswork. It's not faith. It's based on evidence of one's eyes, and the product of one's intellect. It's called "reasoning based
on evidence".
You should try it.