Google's Fuchsia OS Developer Site Debuts (Forbes)

Breaking messaging protocols is just as simple as breaking a function-interface. Challenges to support backwards-compatibility remain much the same for both as well - just look at old protocols like SCSI-3 (which is also just message passing basically) and such. In order to support very old version of the protocol you run in danger of collecting a lot of cruft and bit-rot, just as you do in function-call/binary ABIs.

Binary ABIs probably add to the problem, because there is no 'passing-layer', but that is not the only problem in preserving a stable API. The information you pass, may it be function-arguments, or protocol messages have to stay the same for it to claim to be backwards-compatible/stable, and that is the challenge.