Anecdata disagrees; insurance will reject payouts aggressively. I know someone whose house "coincidentally" burned down while his neighbor had a bonfire, and his insurance fought him for months.
Insurance will fight, but they have not the final decission. Here in Spain they were denying thousands of payouts for car accidents if the car was overdue in their ITV (yearly state mandated revision). But the judge had different thoughts, and they had to pay.
Big difference between refusing payout because of expired ITV and this installation.
If you alter your car its electrical harnas wiring and this creates a short somewhere, resulting in your car burning down ... that is a whole difference situation then missing a mandatory inspection. One is deliberate alteration of a vehicle and one is a inspection of condition.
Most insurances have a lot of protective language (for the insurance company), that often comes down to "everything needs to be code compliant/manufacture original/...".
The bigger the damage, the bigger the chance that insurance companies will fight you. If the above mentioned installation creates a small kitchen fire that cost a few 1000 bucks, they probably are not going to fight it.
But what happens if the above mentioned installation burns down the building, kills 5 people... Do not forget that the other neighbors / building owner's insurance companies, will go after your insurance if you are identified as the cause.
The issue is that if you go to court, and a fire comes from your apartment is from a accident, or a state like this, courts are willing to side with the victim party. But if a fire is the result of your own "tampering" with devices, electrical, etc, there is a very, VERY high chance that the court will side with the insurance provider. Insurances are not blank "do whatever you want, you get coverage" type of deals.