No, but most systems and games made before 1992 or so are.
Some systems are easier to emulate than others.
For example, there are many different NES emulators, but no Jaguar emulators. The reason for that is the availability of information on the NES, and the fact that the NES was a very popular system. As another example, there is only one emulator for the less popular Odyssey . Popularity - The more popular the system, the more effort which is put forth into emulating it. Availability of Information - The more information on the system, the easier it is to emulate.