Business logic is not only about backend. Apps usually have a lot of client-side logic that can be written once in KMM and used on both platforms. See [1] for a high level architecture diagram.

I'm an iOS dev and I've been using KMM on a couple of projects for more than a year now. It's really a powerfull technology which allows teams to move faster, but there are downsides, for example lack of native Swift interop, though there are opensource tools trying to solve this [2].

[1]: https://github.com/Kotlin/kmm-production-sample/tree/master#... [2]: https://github.com/touchlab/SKIE