If you pay any kind of attention to the encounter building guidelines in the DMG, this is simply not true. I don't have enough experience with 5e to know if the guidelines are just completely bonkers at all levels of the game, but according to the DMG, a battle against 4 trolls (frequent in Trollhaunt) is a hard for 4 level 13 characters, and deadly for level 10.
Well, I don't.

I've found them to be rather incorrect fairly often, depending entirely on the time of the battle, the mix of classes, and most especially due to the supposed "doubling" of the XP value due to the number of monsters to determine difficulty. That final one I think does more harm to screw up encounter building than anything, especially once you start reaching PC level 5 and up.
So 4 trolls gives us 7,200 XP on their own, but because there's "3 to 6" of them, their XP value doubles to 14,400. So we're supposed to think that these four trolls are Deadly for a party of five 10th level characters. Despite them having only 84 HP each, and hitting if you're lucky on 2 out of the 3 attacks (but oftentimes only once) for an average of 18 HP damage (against someone like a rogue, who might have 70+ in HP). Meanwhile, that 10th level rogue Sneak Attacks for 30+ damage, the Paladin smites for 20-30 damage, the Wizard drops a fireball on two or more of them for 25-35 damage, etc. The fight will probably go three rounds at most, and you'd be hard-pressed to knock any one PC out unless all 4 trolls bum-rushed a single PC. If they instead found themselves each taking on one PC individually... they'll be on the ground before you know it.
Now... should all the trolls within the Warrens all go rushing the party once they heard combat happening, then sure, things could get ugly quickly (but that was true for the 4E version as well.) But provided the PCs are able to go from one fight to the next without drawing other rooms to them with the sounds of battle... I would maintain the entire adventure would work best for level 5 to 7 PCs, and levels 10+ would find it boring unless many more trolls came out for every fight.
But hey... I freely admit this is strictly my experience talking, and yours (and anyone else's) could very well be different. If it is, and the Encounter Building Guidelines work like a charm for you, that's awesome!