I agree with this having been involved in a project that had exactly this model. Convenient magic but breaks down easily in a complex system, particularly if that system is expected to be extensible.
Typically for Java you need to enforce that that everyone uses the same Executor and that has obvious issues where a third-party library is creating asynchronous work. This might be fine for a relatively small eco-system like server-side Swift currently where we are all using NIO's event loops but as the eco-system gets more complex (hopefully) this will be less likely.
Also I can confirm that such issues are a nightmare as @ktoso mentioned and have definitely taken up more of my time that I would have liked them to have.