Based on other people's experience when a MOV fails it either shorts or remains open for all time.
Unacceptable MOV failure mode is an MOV shorting or opening. That is a potential fire condition as others have learned the hard way. A violation of Maximum Parameters defined in MOV manufacturer datasheets. And is why the fuse exists. Fuse is the last and emergency protection so that fire does not happen. That failure occurs only when MOVs are grossly undersized. Light is a warning that the protector was grossly undersized. Unfortunately, being grossly undersized and failing catastrophically gets naive consumers to recommend that protector. The naive believe protectors are one shot devices. Only grossly undersized ones are.
Normal failure mode for MOVs is to degrade. No fuse blows. No risk of fire exists. MOV has no visual indication that it degraded. No light reports an acceptable failure that occurs when the protector is properly sized.
One MOV manufacturer even defines how to test MOV life expectancy. Spike it with at least 10,000 surges. Its threshold voltage must not change more than 5%. IOW it must only degrade; not fail catastrophically.
Potentially destructive surges are maybe hundreds of thousands of joules. How many joules in that APC? A few hundred? So where are hundreds of thousands of joules absorbed? That question must always be answered to have effective protection. A completely different device, also called a protector, provides protection by answering that question.
Lightning is typically 20,000 amps. More responsible manufacturers provide a 'whole house' protector that is at least 50,000 amps. An effective protector does not stop, block, or absorb surges. Effective protectors connect to what does all (and so effective) protection. Single point earth ground. Then hundreds of thousands of joules harmlessly dissipate in earth. Nobody even knows a surge existed. Surge does not even enter the building. Everything is protected. And the protector does not fail.
Critical difference is a ground wire that makes a low impedance (ie 'less than 10 foot') connection to earth. Wire thickness is mostly irrelevant. Wire length to earth (as short as possible) is critical. Other critical factors are no sharp wire bends, ground wire routed separately from other non-grounding wires, and wire not inside metallic conduit. More conditions that say why APC will not even discuss earth ground.
Best protection for a TV cable and satellite dish is a wire from that cable to earth. Then destructive surges connect to earth without entering a building. Everything is protected. That wire must be low impedance ('less than 10 feet, no splices, etc). Other incoming utilities (AC electric, telephone) cannot be earthed directly. So a telco earths a 'whole house' protector for free (as also required by numerous codes and standards). Everyone has one.
Only incoming wire that has no protection is AC electric. Therefore a lightning strike far down the street enters the house hunting for earth ground destructively via appliances. Where does that energy dissipate? Will hundreds of joules inside the APC somehow absorb that energy? Of course not. Once permitted inside, that current will hunt for earth ground destructively via appliances. That surge is incoming to everything. May catastrophically destroy APC MOVs. Will damage appliances that make a better connection to earth.
Surge damage means that current has both an incoming path and an outgoing path. Protection means that current does not and need not enter the building.
Facilities that can never have damage earth BEFORE current enters. The 'whole house' protector connects current to what absorbs the energy - single point earth ground.
More responsible manufacturers provide these superior devices. Most are names that any 'guy' would know for their integrity. Including Siemens, Leviton, ABB, Polyphaser, General Electric, Ditek, Square D, and Intermatic. A Cutler-Hammer (Eaton) version was selling in Lowes and Home Depot for less than $50.
That protector is only simple science. It only does what a wire might do better. For AC electric, it should be 50,000 amps or larger So that MOVs and protective fuses do not blow. But earth ground (not any protector) defines the protection. A protector is only as effective as its earth ground. Questions about surge protection are mostly about how to install the single point earth ground. To both meet and exceed National Electrical code requirements.
Where does hundreds of thousand of joules dissipate? Destructively inside appliances. Or harmlessly outside in earth. The homeowner makes that decision. Because a protector is only as effective as its earth ground.