- They are stinkin' expensive.
- They had...ahem...unfortunate effects on Einstein's digestive system when he was introduced to them as a baby.
- Even the "healthy" whole grain ones are made from grains that are neither sprouted nor soaked (and the fancy sprouted cereals are in an expensive league of their own), so they still contain a plethora of anti-nutrients that would be neutralized (or reduced) by the soaking/sprouting process.
- Most cereals are extruded at high temperature and pressure into all those cute little shapes. One of the best overviews of the extrusion process that I've read is the article Dirty Secrets of the Food Processing Industry which contains the following summary: "I