I've a couple of years of experience of using Kotlin. In many ways I loved it, and it would have become my JVM language of choice but over time I've started feeling uncomfortable about recommending it.
The language is completely closed and controlled by a single commercial software company and there is no community process involved in its evolution and design. I had believed that this business model of programming language design/ownership died decades ago when everyone abandoned all the bespoke commercial languages/IDEs of the 1980s and early 1990s. The way Jetbrains have scuppered the attempt to provide a reasonable vscode plugin feels like reliving the 1990s where the single company providing the language spec, compilers, IDE and tooling for "their" language and aggressively prevented others from providing alternative tools and compilers.
Also compile times are horrendous - even with modern hardware, the compiler barely seems to be able to manage much more than a few 1000 lines/sec. Again, my age means I remember decades ago when compilers/IDEs advertised their performance in terms of 100,000s of lines per second. This means edit/compile/debug workflow is very painful once your project grows to non-toy sizes.
I also grew to dislike the ethos of the "community" which basically seemed to involve condescension and passive aggressive responses to to any questions regarding the design of language features particularly by Jetbrains employees.
Kotlin is undoubtably a more "modern" feeling of development than Java (or C++ for example) but I'm not sure such vendor lock-in is worth it given many other languages are catching up (including Java) or exceed it. Also I used C# for about a year subsequently and realised that Kotlin stole most of its best features from C#. Languages like Rust, Zig, Swift, etc. are just as "modern".
It's also a bit all over the place. Kotlin seems to be afraid that Java is catching up and so is branching out to Multiplatform.
All in all there are less resources on Kotlin but too many things they want to do. Not only are they building the language but they have to reinvent everything because they don't work on Multiplatform.
It used to be simple - a better Java (so it claims). Now I'm not so sure what the direction is.
The language is completely closed and controlled by a single commercial software company and there is no community process involved in its evolution and design. I had believed that this business model of programming language design/ownership died decades ago when everyone abandoned all the bespoke commercial languages/IDEs of the 1980s and early 1990s. The way Jetbrains have scuppered the attempt to provide a reasonable vscode plugin feels like reliving the 1990s where the single company providing the language spec, compilers, IDE and tooling for "their" language and aggressively prevented others from providing alternative tools and compilers.
Also compile times are horrendous - even with modern hardware, the compiler barely seems to be able to manage much more than a few 1000 lines/sec. Again, my age means I remember decades ago when compilers/IDEs advertised their performance in terms of 100,000s of lines per second. This means edit/compile/debug workflow is very painful once your project grows to non-toy sizes.
I also grew to dislike the ethos of the "community" which basically seemed to involve condescension and passive aggressive responses to to any questions regarding the design of language features particularly by Jetbrains employees.
Kotlin is undoubtably a more "modern" feeling of development than Java (or C++ for example) but I'm not sure such vendor lock-in is worth it given many other languages are catching up (including Java) or exceed it. Also I used C# for about a year subsequently and realised that Kotlin stole most of its best features from C#. Languages like Rust, Zig, Swift, etc. are just as "modern".