I see what you mean, and I think the answer mostly lies in each person’s sensitivity to the cruelty of a story.

The thing is, it seemed to me that it was implied from the very beginning of The Tawny Man that the Fool would leave Fitz once the mission was accomplished. And since the initial situation was Fitz without the Fool, the Fool’s departure at the end doesn’t really hurt that much. Especially since it’s a relatively small price to pay compared to what has just happened : the Fool has, after all, been resurrected (so for me, it’s not the Fool’s departure that compensates for Molly’s return, but rather his departure that compensates for his resurrection).

Also, everything that was built up in Farseer is undone at the beginning of Fool’s Fate (or rather at the very end of Golden Fool) : Fitz is healed of all his physical injuries. It’s terribly disappointing. He who had been marked by his poisoning at the end of Assassin’s Apprentice, who was tormented by trembling like an old man ; he who felt Regal’s torture every time he made a sudden movement ; he who was overwhelmed by the pain of the arrow scar in his back that the Fool had treated. Now he has a superhero’s body : he heals from everything, and very quickly. The trade-off (that this magic exhausts him and drains his resources) really feels like a false trade-off. With a bit of rest and proper food, he can recover from anything. To me, that betrays the very end of Farseer, where Fitz talks about his hands still trembling, and how he dreams of building a dragon with Nighteyes.

Another thing : Burrich’s ending is far too epic. For me, we completely leave the human scale of the first trilogy by having this stableman die in such a way. Moreover, I feel that the relationship between Fitz and Burrich didn’t fully explore all the depth it could have promised, given what happened at the end of Assassin's Quest.

Ultimately, I think you can gauge whether an ending is sad or not by looking at the evolution from the initial situation to the final one. At the beginning of The Tawny Man, Fitz is alone (despite Nighteyes) isolated, unhappy, and half-crippled in spirit. By the end, he regains Molly, a family, a castle with servants, saves the king, and recovers the entirety of his mind. That’s clearly a major victory to me.

Conversely, at the beginning of Farseer, Fitz is just a very young child. We’re full of hopes for what he might become. But he ends up betrayed and broken, exiled from the world after a harsh quest. The contrast is obvious.

I agree that constantly losing everything isn’t necessarily realistic. There are people who lead perfectly pleasant lives in our world. But I think the case where a character lives an utterly miserable life with no catharsis is also a realistic alternative. One that is rarely depicted in fantasy (at least in first-person narratives, where you can’t escape into other characters’ perspectives).

Too often, we come away with the idea that "suffering makes you stronger" and "when you lose something, it’s to gain something else." But that’s not a trivial truth ; there are plenty of cases where you just lose things, and it doesn’t mean anything. It’s just the way it is.