It is indeed a common complaint of many of the chiptune genres that they can sound boring, especially for audiences too old or young to have as much nostalgia for the hardware whose constraints set the genre constraints. The basic NES "instruments" were simple wave generators that were about as pure as waveforms get, plus one simple white noise generator.

I've met critics that even think that "pure" chiptune sub-genres such as the ones that stay within the bounds of physical NES hardware "can't" survive long-term. I think they are wrong both because I've heard some extraordinary things done within those constraints in my lifetime and for some of the reasons that "minimalism" will always be a viable subgenre of "Classical" (no matter how boring some people find that, too). But I understand where that idea comes from of how such a unique set of hardware constraints gave us some of the "purest" waveforms possible and why such pure waveforms are generally an anomaly in music.