Also to date a rock their must be detectable amounts of both the parent and the daughter products, and you must be able to tell that the daughter products came from the parent product in the rock. Potassium/Argon dating comes to mind. Potassium is an element that can be found in quite a few minerals, especially the "K-feldspars" or potassium feldspars. K40 decays to Ca40 and Ar40. Only about 10.9% decays to argon but that is the element that geologists are interested in. The reason why is that there are usually other sources of calcium that make it very difficult to detect the calcium from radioactive decay. Argon on the other hand does not usually appear in rocks as part of the elements that make up its minerals so it is fairly safe to assume that it is a radioactive product. Potassium/Argon therefore is a very useful dating tool. You have an element that can be found in almost any rock and a daughter product that is not. The one concern is that you want material that has not been exposed to the atmosphere, since that can be a source of argon.
The main reason that I can see that samarium/neodymium is not used can be seen by what they call these elements. I know off of the top of my head that neodymium is a rare earth, and I am certain that samarium is one too. A daughter product that is rare is a good thing, that increases the odds that any daughter product you see is a result of decay, a parent material that is rare is a bad thing, that means it is highly unlikely that you will have any product to decay in the first place.
