I was talking about two-way communications. Maybe you can securely isolate a bus that has the insecure->secure side reduced to super trivial communication. But I won't trust anything that runs multi-layer software stack and a complex communications protocol; from what I've learned about software security, there's always a bug or a backdoor that can be exploited to elevate access or just break the secure-side components.
(This is, of course, entirely my opinion. I haven't seen the code of these systems, but looking from the outside, nothing I've learned makes me feel like I can trust their security.)
When I saw arbitrary code execution on the NES in Super Mario 3 done with only controller inputs I decided any notion of software security in a system more complex than, say, a microwave oven, wasn't achievable.
I recently heard an "On Star" ad that proclaimed that they can "slow down stolen cars". Consumers are wowed by features that, by definition, are enabled by unwise security decisions. Manufacturers will cash in. IT security in cars will get worse.
(This is, of course, entirely my opinion. I haven't seen the code of these systems, but looking from the outside, nothing I've learned makes me feel like I can trust their security.)