no, as I said ecosystem is the main drawback. it's being used in the OCapN effort to standardize CapTP between Agoric/Spritely/I think Cap'n Proto too, and it's used in https://syndicate-lang.org, but it's not the kind of thing you'd expect to find a library in any random language for. I'm not sure what the "second-best" thing is. maybe "whichever of the several CBOR canonical forms is what CTAP uses", but IIRC that's specifically not the RFC one, so that's a nightmare.

I don't know about BCS but at a glance it seems like it does a reasonable job, assuming everyone agrees on what the format actually is. it's not self-describing though, so extensibility will indeed suffer