The disadvantages to starting with a test-first approach are minimal. Test code is often repetitive, so it might not expose developers to the entire scope of language features if that's all you're using Kotlin for. If your code is not shipped to production, you won't see how it behaves on different deployment targets. And finally, if you're not already testing, then starting with test-first integration also requires you to rethink your development workflows to take testing into account.
It's likely that none of these disadvantages are deal breakers. For many teams, test-first integration is the easiest, lowest risk way to introduce Kotlin to a project.