The h-index (the number n of a researcher's papers that have received at least n citations) may paint a more objective picture of productivity than some metrics, as your News story 'Achievement index climbs the ranks' (Nature 448, 737; 2007) points out. But for all such metrics, context is critical.

Many citations are used simply to flesh out a paper's introduction, having no real significance to the work. Citations are also sometimes made in a negative context, or to fraudulent or retracted publications. Other confounding factors include the practice of 'gratuitous authorship' and the so-called 'Matthew effect', whereby well-established researchers and projects are cited disproportionately more often than those that are less widely known. Finally, bibliometrics do not compensate for the well-known citation bias that favours review articles.