You will probably notice higher fuel consumption, higher oil consumption, coolant leaks, difficult starting and a rough idle. An old, worn engine is also more at risk of sudden failure by throwing a rod, cracking a piston or cylinder, losing the coolant or oil pumps, etc. Basically if parts are in the yellow and not replaced then your next flight will be gambling whether or not those parts will fail.
It's just a question if sudden catastrophic failure is even modeled to Core Accusim as it is to some older Accusim. Granted, Core Accusim gives you better gradual wear but B-17 Accusim gave sudden cylinder deaths (be that good or bad feature). I've yet to have to witness any sudden engine damage not related to crashing, nose-over, or other external shock on either Core Accusim craft. In reality, something that appears
"green" (in oil pressure, or compression test) can within a fraction of a second to go to "red minus minus". Something like a snapped rod for example, a manufacturing defect that certainly won't be noticed at all in any compression test, nor in oil pressure, nor in engine imbalance, nor in lost power (increased friction, increased heat), nor in metallic particles in the oil... not in anything (aside from possibly a complete engine tear-down)... until a catastrophic failure that can cause a rod to puncture the engine block and at worst kill the whole engine without a seconds notice.
Granted, catastrophic engine failures are damn rare. Aircraft engines are intended to be reliable even compared to car engines and most of us won't experience a sudden engine blow up in our car even during our lifetime.