When I ran a 1-20 level 5E campaign, a modified version of SKT was the heart of Tier 3. I really liked it!
Yes, the published adventure has plot problems. Which is strange, because it really should not. The premise is simple: Giants are on the warpath across the North. Find out why, stop the giants, and get phat loot. Kind of hard to mess that up. And yet, somehow, WotC did.
Fixing the plot requires DMs to come up with their own answers to three questions:
- Why are the giants on the warpath?
- What do they want?
- What is the best way(s) to stop them?
Additionally, DMs should decide which giant lairs and villains they want to spotlight. The lairs are uniformly great and the giant chiefs are good, too.
Giants hit hard and have a lot of hit points. They make great threats in Tier 2 and Tier 3. However, combat with them can get grindy, so DMs should have a strategy for how to mix up combat as the story progresses.
If you pull it off, SKT can feel truly epic. Player characters travel across the North as it's gripped by violence, chaos, and fear. They infiltrate towering strongholds in savage but striking locations, felling mighty giants, and recovering ancient artifacts. It's iconic D&D.
In my campaign, the giants' overpriest, Hekaton, had been captured and brainwashed by the Zhentarim. Under their influence, Hekaton ordered the giants to go to war. The Zhents used the violence to destablize their enemies and gain control of cities across the North. The forces of good were divided and squabbling, each faction beseeching the PCs to come to their individual aid. That gave the players the choice of deciding who to help and which giants to fight. In each giant stronghold they found a clone of Manshoon pulling the strings. Eventually they discovered Hekaton was held in a floating castle. They raided the castle and defeated the dragon that guarded Hekaton -- and then crashed the castle, killing Hekaton! Eventually, they freed Waterdeep from the frost giants that had conquered the city, defeated the original Manshoon, and celebrated as Harshnag became the new overpriest. Good times!