Actually, compared to any other business I deal with, Marriott is far, far, far better in customer service than any other company. These days, dealing with an airline, for example is a joke.
Yes, I have been elite on SPG, Hilton and Hyatt in the past. But I'd take a "normal" Marriott experience any day of the week.
Sure, stuff happens, and it's unfortunate you've had three really bad problems.
I've found that they have a lot of trouble crediting nights when you either check in after midnight or check out before midnight of a night you have paid for.
Second, getting the 500 bonus points to post usually takes a phone call. Almost every stay. There should be a $100 guarantee on that!
Changing reservations - if I did call and make changes over the phone (which I'm not likely to do) I would immediately verify online. I'm paranoid about dates, because I know I can make mistakes - why would I not expect someone else to?
Billy