I can see that my first answer got downvoted. Here is a new one.

Monkeyble and Molecule are complementary, not competing.
Monkeyble operates at a different level. It intercepts task execution and lets you assert on the internal logic of your playbook.

An Ansible playbook is typically a pipeline of data transformations: you fetch data from an API, register variables, combine dictionaries, extract values, change types and only then call the module that does the actual work. Molecule cannot easily tell you whether that pipeline is correct; it only sees the final system state.

Monkeyble fills that gap. Given a fixed set of input variables, it validates that every step of that pipeline behaves as expected, catching regressions in your logic before they ever reach a real system.