This is true for the upcoming best-effort messaging feature (if you set a timeout), and it’s also true for ingress messages currently, but it’s not true if you send messages from a canister, at this time.
More concretely, if the canister calls NNS Governance to spawn and the neuron is indeed spawned and the response is produced, the response cannot be lost. There is indeed no guarantee WHEN the response is delivered. In that sense, it’s possible that the response message sent from NNS Governance to the caller canister is NEVER delivered (extremely unlikely, but possible), but if that happens, I don’t think another call between the exact same 2 canisters will have the responses delivered.
cc @free to keep me honest, especially since this post was referenced. In addition, I think I’m assuming the messages are ordered, which sounds reasonable, but not 100% sure.