What do you mean with “lighter API”? Coroutines alone are not a replacement for Reactor. If you want to compare the both, you would better compare Reactor with Kotlin Flow. The latter has a “lighter” API because it has not nearly as many operators, but the operators make Reactor so powerful. I’d argue that typical processing logic with filter, map etc. would pretty much the same. The main difference would be subscription (consumption of the results), using schedulers, and error handling, but that is not what makes development hard.
However, I don’t want to say that async programming with Reactor would be easy. It takes some time to get used to it. But I’d say the same is true for coroutines and Flow.