I personally opened at least half a dozen cases between August and November 2018. Less than half ever got any kind of response, and those that did always followed the same script, explaining that there was nothing they could do but that my issue should fix itself in x amount of time, which it never did.
In my prior 25 years with Marriott I never once had to open a case. If there was a problem I simply called Marriott and the phone was promptly answered by a competent and empowered associate that would quickly rectify the situation. It really is sad how far Marriott's customer service has fallen.