Bill Nichols’ six modes of documentaries apply to all documentary films. All documentary films belong to a mode, such as reflexive, and so the theory still apply’s to all documentaries, since so documentary is n to poetic, or observatory, etc.
However, the problem with the six modes is that, while they do apply to all documentary films, documentary films do not always belong to a certain mode. They can stray from them, being in multiple modes at different points. This can be seen in The War Game (Peter Watkins, 1965) which operates in performative and participatory at times. This makes it hard to always categorise documentary films into certain, single modes, since multiple may apply to it.
This is because documentary filmmakers need to make their films entertaining to make profit, as well as making them informative and/or artistic. To do this they need to make their film interesting, and this can involve making it it different modes, so it does not strictly follow a simple, single documentary format. This leads to the categorisation of documentary films into specific modes difficult at times.
Finley Cairns A-Level Film Studies Blog 