I think that you are completely valid by stating that you had a good experience with MM+ and the MBs. As with all things, how well something works is partially in the eye of the beholder. You had a good experience, which is great. I personally am happy it worked well for you.
My problem (among many) is that many others are having a bad experience. A highly-rated restaurant where some have great food and others end up being served poor quality food is unacceptable. A good restaurant always NEEDS to be the best they can be. "Off-nights" don't occur in great restaurants, and if they do, guests are compensated. Or a dryclearner who gets some stuff just right but destroys some others' belongings. That is unacceptable. A service provider always needs to provide the same level of service. Disney is such that it can't have the attitude that is ok to do right by some and not by others. Does that make sense? My personal experience with MM+ and the MBs was terrible, and I am not going to be returning to WDW for some time because of it. For the price we pay Disney, they aren't allowed to have "off nights" or anything like that. They always need to be on their A-Game. Always. MM+ was not Disney's A-Game, nor were the MBs able to enhance my enjoyment of my October visit. Quite the opposite.
Let me put it this way. Back in 2010, American Airlines was rolling out first class cabins on some of their American Eagle regional aircraft (on their CRJ-700 fleet to be precise). The first class seats were available for purchase from, I believe, July 1 onward. However, in June of 2010 they began first class service onboard the flights, and you could choose first class seats for coach ticket prices, so the flight attendants could practice serving food and drinks, and delivering a premium service. They didn't sell first class seats during that month - it was all still being sold as coach. I was able to fly American Eagle between DCA and ORD, sit in first class, get the premium treatment - all on a coach ticket. Why? Because American wanted to have their flight attendants practice before they officially rolled something out - because it wouldn't have been their A-Game while they were practicing. I was a guinea pig for American, and they rewarded my by not charging me for the first class ticket (over the coach prices).
Disney has done the opposite. They are getting on the job training, at the expense of many guest experiences...all while continuing to increase ticket prices. What did I get out of their gaining experience for MM+? Nothing except a bad experience. I got no benefit out of helping WDW gain data and experience. As a consumer, that disappoints me. MM+ isn't the A-Game we expect from WDW. There is a systematic issue with guest experiences regarding MM+ and MBs. For a company like Disney, and a premium experience like WDW, that is unacceptable.
Anything less than Disney's A-Game is a failure.