John, this response seemed to have passed me by so sorry for the late reply. I fully appreciate all responses to the content I am putting out so I beg your forgiveness.
In answer to your comment I have never tried merging multiple short exposures to achieve the same effect. Thinking about it I am sure it would work well. In the past I have done the opposite, creating a self portrait with myself in various positions in the same scene. I would like to try the method you mention in a city scene. However, in a landscape I do not feel this method would fit.
I aim to keep landscape images as natural as possible to reflect the natural world that is being captured. Whilst the human eye does not see what a long exposure captures, I feel it still represents the ebb and flow of a particular scene, therefore retaining the purity of the landscape.
To cover your other points. Whilst shooting an exposure of this length, changes in the light is a problem. There have been numerous occasions when I have started shooting with a cloudy, diffused light and finished with a contrasty sunbathed scene. When this happens the only thing you can do is estimate the increase in exposure and reduce your time accordingly. Experience will increase your success in this area. The increasing dynamic range of cameras and the availabile adjustments when shooting RAW reduce this problem significantly.
Using two ND’s can reduce the quality of your image. The main problem is the introduction of a colour cast. This can be solved in post-production or by using the highest quality filters you can afford. Adding ND gradients helps in obtaining a proper exposure, as with any landscape, but this increases the cost, difficulty and convience of shooting a scene.
Overall I find the method I employed here often creates images that are striking and forces people to think twice. In the day and age of 20 second video’s on Facebook I think making poeple think once is an achievement. If you have read this far, I am sure you know what I mean.