It’s the aftercare if the boiler fails, I can only go off feedback and experience baxi/Ideal usually 48hrs, Vaillant/glowworm 48/72hrs Worcester 24hrs not sure on intergas as for ferroli hmmm.
What ever boiler you have will need a call out from manufacture and certain manufacturers engineers don’t help themselves, but Worcester will change anything on the boiler without question comparing to others that will have call out charges/ etc.
As I say it’s only feedback and experience I’ve had so no one bites my head off