A few friends were experimenting with 3D-printed gears for their robots, I've advised to switch to helical gears as these should make less noise, and less prone to wear (since the load never happens on a single teeth, but is transfered progressively). Of course, that creates axial load, which might or might not be an issue (and countered with a second cog), and complexify assembly, especially for planetary gears. But if you try (or already have) them out, let me know.
On our part, the test have proved successful, but we're not looking at tens of kilometers, and mostly print PLA.
To clarify why they aren't used more, their production process is traditionally more complex, but that doesn't happen with additive manufacturing. I'd also guess that the teeth shape makes them more robust than conventional filling, and more evenly distributes force on the layer joints.