It's a downer, but I don't think it's unfitting for the game or even the story (including the books), so I'm not sure if I could call it a 'disappointment' if you end up getting that ending. The books paint Ciri as the great treasure, similar to the Holy Grail or items like that, and searching for them leads Yen and Geralt to danger again and again, with Geralt never actually catching Ciri.
When I think of a disappointing ending, I think of an ending that doesn't fit, one that doesn't act in accordance with the tone, or with the gravity of the choices you've made thus far, or just feels rushed. The MGSV ending mentioned earlier in this thread fits that pretty well. Conceptually it's a fine ending, but it's just thrust upon the character with no explanation or resolution to several other plot points. The Witcher 3 and MGSV were released pretty close to each other, and the Witcher 3 is, fortunately, a finished game.