The biggest problem is that on some models, particularly MacBooks, the RAM is soldered on – making removal near impossible and hazardous to your computer. In some cases the RAM in a Mac is user upgradable, in other cases it is possible to get it upgraded by a service centre, and in some cases (unfortunately) it is not possible to update the RAM at all.
Having estabilished that some extra RAM may help your Mac the next big question to answer is whether you will actually be able to add RAM to your Mac.