As you noted, soil mixes/media vary across the board.
Some of the reasons for adding organic material into the mix include increased moisture & nutrient retention, and sometimes for soil acidification. Junipers & pines don't like to be wet and do very well in drier, nutrient-deficient soils. Maples, azaleas, and some other deciduous trees do better with soil that remains on the damper side, with azaleas in particular preferring a more acidic pH. Juniper roots don't fill a pot so quickly, and can therefore stay in the same soil for several years, decades even. When organic material decomposes, it clogs drainage & causes root rot, which isn't good for any bonsai. Deciduous trees, however, can fill a pot with roots in only a few years. Vigorous growers, such as willow, require repotting more often, and this, by default, means the organic matter is discarded before it breaks down completely.
This is just general guidance. The best advice is to research the type of tree you wish to keep, as you are doing, and learn what works best. There's no magic bullet for soil mixes (except maybe good draining properties). Hope that helps.
