Let's consider fail-live.
If your FADEC sends the signal to reduce fuel flow, your engine is going to idle back.
If your FADEC sends the signal to send fuel flow to max, your engine is going to go to max thrust.
If the FADEC sends oscillating signals to put fuel flow at min, then at max, then at min, then at max,.... with a frequency of X Hertz, then ..... well, somebody who designed the system can maybe tell us what your engine is going to do. Does it depend on the value of X?
Then there are the infinitely varied other signal behaviors that can occur.
PBL
Well, for the first two, that somewhat assumes that this strangely failed FADEC is going to continue to respect the scheduled limitations for idle and max fuel flows respectively. I wouldn't put a whole lot of hope into that, personally. A run down to stop or a runaway to firewall and beyond sound just as likely to me.
The oscillatory behaviour sounds a lot like a bode test, something done to certify engine response. usually that's constrained by input dynamics (how fast can you slam the throttle back and forward?) and by input signal filtering. With neither a constraint here, there's no guarantee anything good might happen. And, again, who says it'll stop at idle and max thrust levels.
I agree 100% with the last sentence, and the sentiment you expressed earlier regarding the difficulty of even the designers understanding failure modes in complex systems, never mind trying to reverse engineer it from a very cursory descripotion in an AFM or operating manual.