Multiprocess approaches are both significantly more complicated to implement and significantly more constrained in their functionality.
Unless you have overriding design restrictions (such as requiring privilege separated worker processes), it seems that the only genuine argument for the "multiprocessing" model is simply that Python doesn't support anything else.
Complicated looking perhaps. But the ERRORS you see are significantly easier to debug.
I come from the "Why not at least look at multiprocessing" in C world as well. Threads are a way lots of very subtle bugs happen. It's a "all is well until someone loses an eye" type of situation.
Both are valid ways to solve problems, but both have their share of issues.
And they're not "significantly" more complicated if you're actually testing your code. Testing multi processed solutions is easy. Testing for threading issues is a pain in the ass.
Unless you have overriding design restrictions (such as requiring privilege separated worker processes), it seems that the only genuine argument for the "multiprocessing" model is simply that Python doesn't support anything else.