For me, getting hooked on the predatory gacha in the last year of UX/DR was a big personal low.
Plot-wise, I was quite disappointed by the reveal of Streletzia's killer being the Darkness inside Ventus. Apart from it feeling a cheap solution to a murder mystery I didn't like how it seemed to separate darkness and evil from people, as Ven didn't even need to have any desire to hurt Streletzia for Darkness to kill her. I was a bit reassured by DR in that respect, as that added psychology back into the mix.
My final contender for worst moment is my original copy of KH1 constantly crashing during the credits so I could never watch the secret ending. Heartbreaking.