I love this thread! It shows that everything is hated by somebody. Some of my personal favorite cameras and lenses are mentioned as disappointments.
So without further adieu (ado, add dew) my biggest disappointments ever in my nearly half century of photography were the Nikon F3 and FM. Coming from several years using the amazing and totally reliable F and F2 Nikons, I jumped immediately to the F3 and FM when first released. The F3 was the most unreliable camera I've ever owned. It just stopped working so often and went back to NPS so regularly (without any remedy), I stopped using it and sold it for next to nothing. I told the buyer of the problems because I really felt guilty selling him that POS. The Nikon FM: The motor drive for the FM was so unreliable I removed it and smashed it on the sidewalk following one of many assignments on which the miserable POS stopped working and jammed up the camera. Without a motor, the FM was more reliable but never felt like a "real" Nikon--light duty and cheap feeling. As for the smashing incident, I never felt better about doing something so totally wrong in my life. It was a mercy killing.
I've been sorta-kinda disappointed with Olympus Micro 4/3 gear. I've owned a fair to middling bit of it over the years and I've done some fine photos with the cameras. But I made better photos with the old Olympus Standard 4/3 gear that Micro 4/3 replaced. Why, I dunno. I guess I bonded with the old DSLRs more. Despite that, I still shoot sometimes with an OMD EM1 on occasions, mostly with adapted Oly 4/3 lenses. The results are always excellent but the shooting experience doesn't really please me. Can't really put my finger on it.
The only other disappointment I can recall were my attempts at using large format cameras. I owned two 4x5 Wista outfits over the years (the first was stolen and replaced by the second). Nothing wrong with the cameras or the Schneider, Rodenstock and Fuji lenses I used but I just never really felt comfortable using them. At the time I shot with them, I also carried a Leica M4-P and I always ended up with better photos with the Leica. Not technically better but just more interesting photos. I guess I'm a snapshooter at heart.