Root cellaring of one type or another is probably the lowest tech way of preserving food, and particularly in mild climates probably the easiest; a hole in the ground can successfully store some crop in a cool climate.
However, what about nutrient loss?
Canning loses quite a lot of nutrients, freezing keeps most of them, dehydrating is somewhere in between. Nobody talks about long term root cellar storage. The closest think I can find talks about short term refrigeration, which involves relatively rapid nutrient loss. Can this be extrapolated to root cellaring, or not? If it can, does the process slow down after the first week or so, or not?
If nutrient losses in cellaring are large, than investing in freezer storage and backup power starts to look advisable for those planning to subsist on homegrown food during cold winters.