The first few times I heard it I really didn't get it and some quite good musicians (non-guitarists) I knew at the time didn't much either - compared with the Walton Bagatelles for example. After I'd studied the score and then actually learned it - this being 1992 - I came to have the profound connection I still feel with it today.
David Starobin, at the Bath Festival we both attended Adrian (1995?) said the weakest bit is the Passacaglia. Its true that its as though Britten didn't think he'd be able to write a proper bass theme variation aka passacaglia, in terms of instrumental practicalities as a non-player (Rodrigo and others managed quite well) but my feeling is that despite this he worked around the situation and built a final variation that works in its own terms, albeit not a proper passacaglia as found in other of his works (e.g. violin concerto) and evidence of his connection with Purcell.
If there is an extra-musical point to the piece it is surely that it is, like the text of the song, a meditation on the connection between sleep and death; both in its descriptive, almost programmatic section titles, and the dramatic shape of the whole. It is serious and of course somewhat scary stuff, and I wouldn't be surprised if some shy away from the Nocturnal because deep down they are uncomfortable with the almost psychiatric, at least psycho-analytic sub-text. Certainly, your suggestion that the composer might have occasionally materially lightened the mood, would have ruined the whole point. In fact, there is variety, but it is subtle and within a narrowly focussed range (my copy is not here so point to particular sections).
Where I would earnestly suggest you are making an error is to use terms like "religious infallibility" and "pedestal". If there is anybody, anywhere, who thinks they ought to admire it and go around lying that they do when the don't, they have a bigger problem than you have with the Nocturnal. Those of us who place it in the top handful of guitar compositions do so because its our opinion - however we have arrived at that doesn't really matter; but for instance, if my college tutor Gilbert Biberian had persuaded me I ought to admire and like it, and as a student I managed to despite myself, nearly 30 years later I would hardly have been as thrilled as I was by Sean Shibe's rendition on his recently discussed CD, or to have written that first paragraph. So i suppose I would quietly suggest care to avoid any possible suggestion of denigrating the motivations of others who have different musical tastes and experiences.
I think I've aired this before - but then we've been here before so why not - but it seems to me some people, like my big sister, have a sensitivity to dissonance and general unpredictability that mitigates against an appreciation of the likes of this piece. Others, like myself, enjoy and indeed thrive upon dissonance, complexity, irregularity and quickly get bored with too much of their opposites.
Simon Ambridge Series 40 (2005)
Trevor Semple Series 88 (1992)
Louis Panormo (1838)
Alexander Batov Baroque Guitar (2013)