Looking at is scientifically, with a bent to reach the "perfect" balance (whatever that is), the problem with most organic fertilizers is that they are nutritionally incomplete. Often, one will be fine for "a, b, and d" and another will be good for supplying, "b, c, and traces of x, y, & z", but then you are left attempting to supplement the remaining minerals, and the balance the products used to give the Goldilocks "just right" amount of the minerals. I will grant that, as orchids are not particularly "hungry" plants, getting the diet perfect isn't essential, but making it get close can lead to overall healthier plants.

Then there is also the fact that the ammonium ion, for example, absorbed by the plant is identical whether it originated in guano, mineral potash, or the petrochemical industry.