i've been writing kotlin since 1.0 (2016) and my past few jobs have been at startups using kotlin entirely on the backend. it's really enjoyable, there isn't another language with as good of a development experience due to Jetbrains controlling both sides.

coroutines are the biggest downside imo. they're great for android and other environments, but now that we've got loom on the jvm they're needlessly complex (accidental blocking calls, coloring, headaches with libraries that use thread locals, reentrant lock, etc.)