The difference is that you can look at function, see it takes no nurseries as parameters, and know it does not spawn any background tasks. And if it does take nurseries, then you know it's likely that it does.
> Much like you know that control flow will come back to the function you're reading after calling another function.
What about continuations? Exceptions? Aborts? setjmp()? I can think of many cases where control doesn't return to the caller that are perfectly valid.
Basically, either the code is so simple theres obviously no bugs, or the code is so complex theres no obvious bugs. I feel a nursery is closer to the later than the former.