You're right in that emulating the NES "good-enough" for most games is no big feat, in terms of power, but you are wrong in saying that it's because the platform is "old". There are old systems that are much trickier to emulate simply because software takes advantage of every corner case in the hardware design, using cycle perfect timing to exploit unexpected behavior in the chipset. The C64 is the perfect example, still having a pretty vibrant demoscene, coming up with new tricks that break emulators every year. Then, to get a reasonable level of accuracy, you need to emulate everything per cycle, complete with a bunch of analog hardware (simply emulating the SID sound chip is quite a heavy task) and registers bound to pins simply left floating.

Unless you get into speedrunning games where accuracy matters a lot and some emulators are "banned" because they are lacking in it. Banned in this case of course only meaning that you will not get recognition from most of the communities for performing speedruns on such emulators.