Allow me to elaborate a little on the potential for ambiguity. Consider the phylogeny (A,B)(C,D). What would the grouping of A and C as one taxon be? Paraphyletic or Polyphyletic? Difficult for me to tell, but I know the taxon is not monophyletic. There are also multiple definitions of para and polyphyly that depend on different criteria including whether the homoplasy used to group the non-monophyletic taxon is the result of convergent or parallel evolution. Distinguishing between convegent and parallel evolution can also be difficult and there continues to be debate about this. On the other hand, whether a taxon in monophyletic or not is easy to determine while looking at the phylogeny.

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