Originally Posted by Seattlenerd

I was easily able to cancel a reservation after combining my accounts for a Marriott stay that had two Chase VISA free night certificates to it. Out of caution, I called in to do it (versus doing it online) because of the certs. But it was able to be cancelled quickly, and the phone rep was able to almost immediately re-deposit the certs into my account, though she had to work with a supervisor to do it.

So the inability to cancel a reservation is not universal. As a matter of fact, I can't recall it being reported more than this once, though I may be mis-remembering. Much more common is the inability to see upcoming reservations on the combined website; that's better in the app, in my case.