N/P pellets are part of the evolution of carbon dosing and the first "solid" vodka method. Carbon dosing in some form is key to Zeovit and other UNLS systems. Vodka was a cheaper alternative. Early criticisms of dosing vodka alone was that it would produce a monoculture of bacteria in your aquarium and limit the diversity of good bacteria. Vodka is not the only carbon source. A variation that incorporate vodka-sugar-vinegar and incoorporated biodigest became quite popular as theory of the VSV method was to avoid a monoculture. Here is a good read.
http://glassbox-design.com/2008/achi...perimentation/
As far as I know the make-up of N/P pellets is still unknown so some critics fear it will create a monoculture of bacteria, especially after longterm use.