My intuition is that most of the difficulty comes from the complexity of the individual cells- we don't understand nearly all of the relevant things they do that affect neural firing. This is basically independent of how many neurons there are or how they're wired, so I expect that correctly emulating a nematode brain would only happen when we're quite close to emulating larger brains.

If the "complicated wiring" problem were the biggest hurdle, then you'd expect a long gap between emulating a nematode and emulating a human.