Also would like to hear from someone with some deep experience with it for a good comparison. However, my 2 cents from what I've seen...
You need to include a 1.3Mb kotlin.js library file with your project which isn't ideal. And if you want to use extra features, like the kotlin html-js lib, you also need to add those, which further increases the JS download size.
Probably a good choice for connecting a Kotlin project to a JS project, or if you just need some simple JS for your Kotlin project. If you needed more features, GWT would be a better choice (with its UIBinder, RPC, ...).
I agree with your comments about J2CL.
Personally, I'm more excited about Java to WASM compilations that will hopefully be more supported now WASM is getting garbage collection.