MarkB
Legend
I can somewhat corroborate that - if you do start with your PCs a little higher (as I did) - many of the later encounters, especially the random ones, become somewhat of a cakewalk for the heroes.
I really ramped up (often increasing the amount of, or adding additional monsters) those encounters, but many of them were still quite the yawn for my players, as they buzz-sawed through them.
The last section I ran my players through was Blingdenstone, and I found it fairly hard to challenge them even there. In a lot of cases they were up against only one or two opponents, and unless I did a lot to pad things out with random encounters, they often only had one or two major confrontations a day.
One particular encounter was absolutely ridiculous in the end.
[sblock]They heard about the Medusa early on, and then had a scare when one of them was petrified (no pun intended) by a basilisk in Entemoch's Boon - so despite a couple of them really wanting that shiny big diamond, they managed to sneak around Rockblight and steer clear of her until after they'd done everything else there, including defeating the Pudding King (who was also, quite frankly, a pushover).
When they finally went back for her, I tried spicing up the encounter by adding a couple of gargoyles to the mix, and tarting up the medusa's lair with some statuary and a waterfall for her to emerge from behind - but it was still ludicrously simple.
The warlock cast Darkness over the entire party, which removed the need for anyone to avert their eyes. The way darkness works in 5e basically means its penalties cancel out (characters suffer disadvantage for attacking someone they can't see, and gain advantage for attacking someone who can't see them) so everyone was essentially hitting at full strength while negating the medusa's main power.
The medusa stayed back outside the darkness and took potshots with her bow, but the players had planned ahead, and had used their previously-acquired Stone of Controlling Earth Elementals to summon an Earth elemental just prior to entering the chamber, plus they had Glabbagool with them. So while they kept the gargoyles busy, the medusa was confronting one opponent who didn't use eyes to see, and another one who was already made of stone and could also see without eyes if necessary using tremorsense. Glabbagool engulfed the medusa and got a full round of dealing massive acid damage to her before she managed to use her +0 strength to wriggle free, after which the elemental pounded her into goo.[/sblock]