Radiometric dating is a very useful tool, but it does have limits: The material being dated must have measurable amounts of the parent and/or the daughter isotopes. Radiometric dating can be done on only some materials. It is not useful for determining the age of sedimentary rocks.

Is there an upper or lower limit to radiometric dating?

The reason it isnt 1 million year old is because the half-life of 14-C is about 5 730 years, which means after about 50 000 years there is no more 14-C to measure, hence the limit of that dating technique is about 50 000 years. All different decay series have upper and lower limits for which they work effectively.