I know progress has been made decoding the nerve impulses directly for vision and sound, to the point that we can engineer machines that generate the impulses (pretty much the hardest test there is), with Cochlear implants being standard, off-the-shelf medical tech now [1]. The eye equivalents are harder, but prototypes are being built. (I don't think they're off-the-shelf tech yet, though.)

I have not heard anything similar about the olfactory nerves. I suspect they're going to prove to be much messier. Perhaps not necessarily "complicated" in some sense, but messier. But it shouldn't be impossible.

[1]: https://en.wikipedia.org/wiki/Cochlear_implant