Couldn't that also potentially break some logic branch through the chip where the app is expected to be running on a GenuineIntel processor? I'm not well versed in this.
If code is dumb enough to try to use something low-level (let's use Bull Mountain RDRAND as an example) without checking for that specific feature bit, then it obviously is the code that is broken, leading to an illegal operation and it gets killed. That's not the CPU's fault.
Intel and AMD CPU manuals both pound in the point, too. In the sections on these advanced features they always insist that you check the feature bit first.
But more likely, it would mean slower code execution, as the code path wouldn't be optimised for AMD:
i.e. if the processor check thinks it's running an i7 Sandy Bridge, with an SSE float divide latency of 11, but the AMD chip has a latency of 23, so the unrolled loop that worked well for the i7 doesn't work at all for the AMD chip.