If you've ever seen an emulator mature before you'll see that there are two types of games that take the longest time to become perfectly emulated, obscure games and games which use the system to its fullest. For the most part I don't think people are worried about obscure games ... Now, many of the best (or at least best looking) games on a system use all of the features of the hardware to get the best performance out of it; at the same time these developers will look for little tricks in order to get optimal performance out of the system. Some developers have found insane things (like an ability to run a process on a sound processor/input controller) that are nearly impossible to predict ... As long as they don't have perfect emulation there will be several of the best games left off of the compatibility list; on top of that I would expect that if they have 50% of the games emulated then less than 50% of good games will work.