I do not have an opinion on if Kotlin is great as a backend for every project. Kotlin does excel as a pick for projects where you are also compiling for native mobile front ends.

Write a Kotlin multiplatform client side business logic module in tandem with your Kotlin backend. The multiplatform module compiles for both your Android and Apple environments and for extra flexibility you are able to quickly port code from the client business logic module to the backend (or vice versa).