I generally like storm king's thunder but it suffers from meandering plot syndrome.
It tells a great story that the Pcs aren't really involved in
How do you fix?
It's not a meandering plot. It's a late-appearing plot.
The thing with SKT is that it presumes that the characters are taking part in a D&D campaign where the players are wandering around the Sword Coast, doing small quests for people. (Much as in the D&D Essentials Kit).
Into that campaign, SKT slowly brings in the idea that giants are causing trouble. "Giants attack a town" could be just another small quest, much as what has occurred before. But then you leave Triboar (or wherever), and the players begin to realise that there are more giants about than there should be. And they're doing odd things.
At this point, the characters are still doing small quests, but they're realising something is wrong in the world.
And then Harshnag turns up, and the plot gets underway.
The trouble with "A Great Upheaval" is that it starts things too soon. The players get to know there's trouble in giant land at the very beginning, rather than working it out for themselves. And then the adventure abandons that plot thread to go back to "general adventuring" and the players wonder what is going on.
Ignore A Great Upheaval. It's a disaster. Run a standard D&D campaign for the first five levels in the Sword Coast, then slowly build the giant threat... and then let the rest of the adventure proceed.
Cheers!