A skilled operator may be able to construct a mask from information taken from several channels, but this is way out of reach of inexperienced and novice users. The general principle is to find enough contrast in one color channel to define the edges and areas of transparency in the color image. Can be attempted with any image, however there must be enough available information in the color channels of the image for the process to work. This usually makes it useful for only the most limited applications. Very fast .
All images require the same amount of user effort.
Only file size affects the time required, but of course, that effects all of the methods. Very, very slow. Once again, can be done by a chimp . Extremely dependent on skill level of operator. Not for the faint of heart.