Huh. I'd be interested to hear what low-level stuff they thought was harder than in Java. Can't think of anything you can do in Java that can't be done at the same abstraction level in Kotlin.
And bitwise operations https://discuss.kotlinlang.org/t/when-does-bit-fiddling-come...
And maybe handling generics. I haven't had any issues, but kotlin is a bit stricter which is generally a good thing, but I'd believe that for some edge case it makes things harder.