Has radiometric dating always been accurate?

The quick answer is no.  As mentioned in the article about assumptions, when test results do not measure in accordance with the expected long ages, then the specimens are discarded and assumed to be contaminated.  A 200 year old lava rock from recently erupted volcano has been measured using the potassium-argon method.  The ages ranged from 1.6 million years to 2.96 billion years.  Moon rocks were measured with many different tests, and those results ranged from 2 million years to 28 billion years.   More information about assumptions and inaccurate dates can be found at this website.

One quote to remember: When the age of rocks are known, the radiometric dates are incorrect, but when the age of rocks are unknown, the radiometric dates are assumed to be correct.