I've only had two instances where a hotel was unable to accommodate my reservation. The first time, they booked me in an room at the nearest hotel with space, which happened to be a competitor and an upgraded room. The second time, the hotel booked me into the largest room they had available, which happened to be their Presidential suite. (I booked through my credit card's "The Edit" program.)

I've had several AirBnb hosts cancel. Airbnb provided an alternative option the first time but the second time did not. Luckily my credit card's travel insurance treated it like a hotel booking and was able to secure a reservation at a nearby hotel. And that was the last time I used AirBnB.