Let's be real; most supposed documentaries about the making of a film are little more than an advertisement for the film in question. There have been the exceptions of course, but the majority have been more along the lines of infomercials than anything else.
Think about any of the "making of" featurettes that are included as part of nine out ten DVD packages these days, and what, if any, information they, actually give out about the process of making the movie. Oh sure, they'll tell you all about how ingenious the special effects were, and you can count on a couple of "on set" interviews with actors in costume and makeup talking about how great everybody is, but what have you actually learnt about the story behind making the movie?
If you're actually going to document the making of a movie, you can't just pop onto the set for a day or two and do a few set pieces with actors and crew - you need to be able to stick your camera into every nook and cranny of the filmmaking process. Yet even all the access in the world won't give you an interesting movie without there being a story beyond filming the filming. Having sat for hours on end waiting to appear on camera for two minutes, I know from personal experience how boring modern movie-making can be.
When film maker Jon Gustafsson was cast by Sturla Gunnarsson to play an anonymous member of Beowulf's team of soldiers in the Canadian/Icelandic/British production of Beowulf & Grendel, he brought his camera along to make a record of the events. However, I seriously doubt he could have known in advance that he would have ended up with Wrath Of Gods.
Knowing that shooting was going to be entirely on location in Iceland, he would have known that he would have spectacular vistas to use as backdrops for whatever footage he took. But there is no way he could have known that making Beowulf & Grendel would turn into a quest so fraught with difficulties and dangers that it would demand its cast and crew have the fortitude of Norse heroes to complete the picture.
Director Sturla Gunnarsson's original idea was to take advantage of the extended daylight hours of an Icelandic summer to ensure as short as shooting schedule as possible. But almost from the moment they arrived in Iceland things were thrown into chaos. The production manager who had drawn up their initial budget had never worked in Iceland, and had badly underestimated the costs involved with working in the most expensive country in Europe. Compounding the problem was that the Icelandic kroner decided to shoot through the roof and rose 20% in value against the dollar. The movie was over budget without a frame of film being shot, and without its financing in place.