Kenko DGX 2x Teleplus Pro 300 with 60D

We probably should talk about "below f/5.6," because we're talking about a smaller aperture than f/5.6.  Anyway, although I don't understand the technicality of this, the practicality is that the AF on a crop-sensor Canon camera works poorly if at all at smaller maximum lens apertures than f/5.6.  I think it has to do with the geometry of the light circle and the sensor.  Canon lenses report their maximum aperture to the camera and if it's smaller than f/5.6--which is what would happen if you put an extender on the 100-400 mm Canon lens--the AF is disabled.

For manual focus, you need light.  The less light you have, the harder it is to see if the image in the viewfinder is sharp or not, which is how you have to manually focus.  If you put a 2X extender on a Canon 100-400 f/4.5-5.6L lens, you get an 800 mm f/9-11 lens.  f/11 is pretty dark, so pretty hard to manually focus.

In addition, as other posters have pointed out, the Canon 100-400 f/4.5-5.6L lens doesn't really perform well with extenders.

FF