I'd say the language level interop is "very nice". Library level interop is still a work in progress. Many (most?) facades are incomplete, and the more complete ones often eschew compatibility with existing JS code in favor of optimizing for scala-js only. Typically figuring out exactly what the possible argument and return values are for a given js function takes a bit of trial and error, so you have to twiddle the facade a bit and be prepared to fork them and contribute changes back upstream.
That said, if you're familiar enough with both platforms to handle an app that has both, moving the interop from your head to the scala.js code isn't that big of a leap.
That said, if you're familiar enough with both platforms to handle an app that has both, moving the interop from your head to the scala.js code isn't that big of a leap.