I think the build-up was very good: we were learning significant things, visiting important, mysterious places, and the pacing was good. I particularly enjoyed: getting to learn more about Vlast only after he died, being lost and just asking everyone around for what to do next; Balthazar actually killing you, which made him look dangerous enough even for your plot armor; using Palawa Joko's undead army for you. But then the ending was anticlimactic.
First, the final battle wasn't as epic as the Mordremoth fight, or even the Zhaitan fight. Suddenly with a sword (which Rytlock had been carrying with him all along) you are so much more powerful and enemies melt before you. You don't really get the feeling of fighting against impossible odds. And Balthazar lacks any character, even more than other standard villains. He's a human god after all so you'd expect he at least has some coherent motivation other than just "being evil because he's a villain," but no, he doesn't have any. His manners don't even feel dignified enough for an end boss. Vlast's inner conflict that we learn about from the crystals and in Kesho turns out to not amount to anything. The fact that we end up siding with Kralkatorrik (though only out of convenience) and the mention of a previous attempt at "converting" Kralkatorrik suggest there could be some interesting interaction with Kralkatorrik but we never get any.
Overall, the PoF story feels like it's just a prelude to the inevitable fight with Kralkatorrik coming in LS4, with Balthazar being a complete filler villain, a mid boss rather than an end boss. It sets up a lot of things, which I suppose LS4 will expand on, but the Balthazar storyline itself was very disappointing.