Well as the thread is 'Games that let you down'. I'm inclined to think 'computer games' and not waste a lot of time specifying particular examples, as I think so much of the problem is computer games, and that includes consoles, as a console is just another form of methodology of delivery, much like a car and a truck are just forms of conveyance with specific ways of getting the objective accomplished.
When I was a kid, playing games meant playing them with people. WITH PEOPLE MEANT THAT THEY WERE ACTUALLY PHYSICALLY PRESENT THOUGH!.
Modern life though has resulted in so much of society accepting that playing a game solo was acceptable. Some of us simply don't agree though.
I have no real background with MMOs simply because my background is with pen and paper. I've watched them for a long time, I know they well enough from having friends that play them and my son plays them. There's nothing really special about them though, any more than some cars are Ladas and some are Porches and I am not impressed by being told how great a Ford is.
They removed the one aspect from the experience that mattered. Being with friends.
I would never have gotten interested in wargames, if my first encounter had been with one on a machine though, and that includes every wargame we've ever seen. My first taste was Tactic II and my reply was 'wow, almost like chess, but so much more detailed'. Then I discovered Squad Leader and Panzer Blitz and Third Reich. But they were all played with local friends. And Squad Leader will always be the ABBA game as my regular local friend opponent was Swedish and we played a lot of ABBA when we gamed. Third Reich was also impacted by the way my silly friend would act all ridiculous while being the Germans. He usually played the Germans so I tended to act very silly as the Allies. To listen to us, you would have thought we were completely nuts.
None of that exists with games where it is just you and the machine. I actually feel pity if you have come to LIKE being just you and the machine and a dreadfully stupid opponent. You might not like my pity, but you are going to have to deal with it, as I am not planning to stop pitying you willing loners any time soon. I'd probably pity you even if the AIs were good. Actually, I likely would, as you would just be more convinced that a human wasn't as much fun.
Humans are social creatures after all. I enjoy doing anything more so when I have company. It's more fun to shop with someone, it is more fun to bake with someone, it is more fun to watch a movie with someone. I even like reading a book knowing someone else is in the room.
I have often felt sad, that we have used our computers more as companions, than as tools. Because, there is really no reason that our computers are not being used as bridges to bring board games a bit closer together. It's sad that Vassal is not more enthusiastically embraced as an idea. I also wish the virtual table top would hurry up and evolve faster. I'd love to enjoy the graphical enhancements instead of all the paper clutter at the table getting in the way of the minis and the books. I think it would be cool if the gang was all lounging in chairs and seeing the game appear on the hosts 40+ inch TV screen.
I don't want my computer to be my opponent, I want it to make it so my table is not required, so that the game board is the TV screen and neither I nor my friends have to wreck our backs constantly reaching over a map that is measured in feet not inches.
So yes, if you want to know, 'what games have let you down?' I'd be saying digital games. They have failed humanity, in allowing us to not require humans to play them. It's like listening to people explain the virtues of playing hide and seek alone and how Solitaire is more fun than a poker night with the gang. Sorry boys, it doesn't wash, and for those that think it is a great thing to not need to play against other, you have my pity.
I have too many too complicated wargames, and not enough sufficiently interested non wargamer friends.