Some systems are much harder to emulate than others, because the developers of the hardware used sneaky tricks to copy-protect games.

For example, a Playstation game is a regular CD with bad sectors burned into it.
There are channels of data on a CD called subchannels, which contain the information necessary to match the audio up with the video, so that you get the correct sound at the correct point in the game. For parts of the game which are silent, this data obviously doesn't matter, because it doesn't need to match up audio with video. Often the section of the game that they use is at the start, when the developers logos are on screen. So the makers of Playstation games burned the subchannel data for the silent bits as random nonsense, something which conventional CD writers are incapable of. When the Playstation boots up, it checks the subchannel data for the disc in the machine, and if it makes sense, it must be a copied game, and won't play.

It's because of tricks like this that emulation software can't be fully capable of emulating the hardware properly. There were actually no PSX emulators capable of emulating copied or ripped games until the author of ePSXe devised a workaround for subchannel reading. It would defeat the purpose of an emulator to only emulate authentic games.