Is the XRF spectroscopy equally sensitive for every element?
In a very very broad reasoning, XRF is much more sensitive to the "mid-range" elements. 

Elements below Na are generally not detectable. 


The x-ray tubes must have a beryllium window to seal the tubs which absorbs the low energy x-rays.
The lower the atomic number the less the fluorescent yield. 
Low energy x-rays don't penetrate from the sample efficiently. 
Low energy x-rays tend to be absorbed in the detector windows. 


For the very heavy elements the detectability is poor too. Say anything about lanthanum.


The K x-rays are so energetic that a high voltage on the x-ray tube is needed to excite them. 
The lanthanides and actinides tend to behave similarly chemically so geological samples tend to contain a mixture which creates peak overlaps.
Higher energy detection of x-rays is poor. EDS because the x-rays penetrate the detector. WDS because the available crystals don't diffract high energies well. 
The L x-rays are not quite as useful because the greater number of x-rays leads to more peak overlap.