I couldn't give you an ironclad guarantee at my house. It's almost always quite good but it goes down from time to time. Major hotel chains are pretty good these days (generally). B&B's/small inns are pretty hit or miss.
Rather than an ironclad guarantee that the internet will 100% stay functional, more precisely what I want is a guarantee that the host will be approximately as responsive to outages as I myself would be were it my own internet at my own home, which is the best that anyone can realistically do.
I suppose if someone’s internet went down for weeks I might have to opt out of the reservation, which isn’t totally fair to the host if it’s not their fault and it’s beyond their control.
But also that has never happened, and it similarly wouldn’t be totally fair to me if the internet were to go down at my own home and I had to rent a coworking space for an extended period.