When I arrived late in the evening my room was not available: the (online, credit card backed) reservation I've made was "lost" and no more rooms available.
There was no way to sort it out easily so I booked another Hotel (higher price, and had no time to do some Tripadvisor/Rating research).
The exactly same thing happened when I booked a hotel in Czech last year via HRS. When I arrived I was told by the hotel guy that there was only 1 room available instead of the booked 2. I tried to sort it out directly with the hotel - no chance after 10 min. of discussions.
I then called the HRS hotline, the service agent put me on hold while he called the hotel. I saw the guy at the reception pick up the phone, a few seconds later he came to me and told me that 2 rooms were available. :/
That said: it seems that the OTAs (HRS, booking, expedia) have enough "arguments" (e.g. threaten with delisting etc) that as a customer you may also have benefits from this kind of oligopoly.