From the mind of Iggy Koopa~ 
I haven't hacked many games, but I'd say Yoshi's Island has a very strange format. There are more than 255 sprites so each number can give you one of two different kind sprites, one for an even, one for an odd sprite axis. The game also has unreasonable object stuff. 5 byte objects are reasonable, but 4 byte objects are just mean. There are like 509 4 byte objects, since object zero itself has 255 types, which uses a different format (00 YX screen, YX Location, Type, rather than Type, YX screen, YX Location, Length). And the Levels are scattered throughout the rom. The first level that appears in the rom, starting from 000000 is at 6000 and ends at 8000, then the next one begins around 7B000. And there are two other levels way outside the regular range (it's right in between text data), and worst yet, the pointer table's pointer for that level doesn't even point there, it points to some address that doesn't exist in a 200000 byte ROM. But besides all that, its a rather easy rom to hack.