It seems like a lot of the complexity may also come in when you have important circuits to emulate other than the CPU. In that case you'd have more to worry about in terms of timing, dataflow, and synchronization. And some platforms might conceivably have analog circuits that play an important role too, although I guess you might be able to abstract that out by trying to create a rough digital functional equivalent even if it isn't completely faithful to the behavior of the analog part.
that's true! but I still think you can get a lot of the way with a NeoGeo emulator without it being perfect on timing. that's a lot of the reason emulators work well on most games but fail to replicate the right graphics or sound in some - some games played hardware tricks to optimize.
