Inconvenients
Yeah there has to be some ;¬)
Difficulty of application
Unfortunately, this technique doesn't apply very easily to any kind of image. For instance, if you use it with photographs, there is very little chance that you'll get any size reduction, since a photo barely ever features exactly repetitive areas. Nevertheless, you can still use the scrolling, as shows Pat McClellan, in this article on DOUG (sources available)
The tile based scrolling specs will be better used on images completely created from scratch, for the sole purpose of being used with this technique. Which leads us to the second inconvenient...
Difficulty of creation
To create a level map, you'd better do it "upside down", that is start with tiles, and create the image with them. You'd then still have to create the map, which