This module is severly undertuned for a Tier 4 module. Aside from perhaps the final boss, I suspect the module could be completed on average difficulty by a 10th level party without much trouble. When I ran it, I offered the table the option of having me running it as written (with the caveat that I believed it to be undertuned) or to attempt to upscale it appropriately The table opt'd for running it as written which led to us finishing the ostensibly 6 hour module in a little under 2.5 hours (I ran a second module with the remaining time). I do not believe that a single character in their party was ever reduced below half hit points and nobody ever felt threatened. I will note that I ran this as weak due to the party being entirely level 17 and only having 4 members. I do not believe that it would have been suitable even if I'd run it on strong.
There are two major problems with it as written:
1) There is no inherent time pressure, so short resting is free.
2) Every single fight is too light.
Bonus Objective A: No number of CR3 gith warriors represent a threat to a tier 4 party. They mostly can't hit and even if they do, they do pitiful damage. Further, since this fight occurs before the "core" of the module, there is nothing to prevent the party from taking even a long rest after doing it. [Possible fix: Upgrade warriors to knights; knights to gishes; and gishes to 2x gishes]
Fight 1: 2x Zombie Beholder; 2x Shadow Assassins. 2x Zombie Beholders were a Tier 2 fight in Season 7. The shadow assassins are certainly dangerous--but only due to strength drain. The party went through with lights on (a glowing Light Cleric) and so it the assassins were largely negated. Most parties won't have 100% darkvision, so this fight is rarely going to "come together" into a threat. [Possible fix: replace 1 or 2 Zombie Beholders with normal Beholders]
Fight 2: Larval Mage + Cadaver Collector. Finally enemies that are almost the correct CR. Here, the main problem is action economy. Even a weak party will have more turns than them. My party's monk stunned both of them before they had their first turn in combat, so they never got turns. The only resources the party spent on this fight were ki points. [Possible Fix: more-mid CR fodder]
Bonus Objective B: CR3 and CR4 Deathlocks; CR5 Flesh Golems; CR6 Mage. This is just awful. The only enemy that got a turn in combat was the flesh golem and that's because its a sack of hit points. The deathlocks died to a single fireball. Again, these are completely tier-inappropriate. The Mage + Fleshgolems optional fight is similarly trivial. [Possible fix: Replace deathlocks with Warlock of the Fiends with max hp]
Fight 3: Boneclaw + Shadow Assassin. Boss fights mostly don't work in 5e. This is an easy example of that. The party's crossbow expert was sent ahead to scout because he had a cloak of invisibility. He was spotted by the boneclaw. He solo'd the boneclaw in two rounds without action surging. [Possibile fix: probably needs 3-4 boneclaws]
Fight 4: 3x Death Slaadi. The module description notes that the characters are automatically aware of the enemies, so no surprise. Even if they are invisible, 3x fireballs aren't that scary. Only one of them managed to get their fireball off (the other's invisibility was dispel-magic'd and so the spell was countered). [Possible fix: more death slaads or give them surprise]
Fight 5: 2x Gibbering Mouthers. I don't understand why this is here. Gibbering mouthers are CR2. Two of them show up as part of the final fight in a Tier 1 CCC. This isn't even a speed bump. [Possible fix: just remove this encounter, it is a waste of time]
Fight 6: Lich + 5x Flesh Golems. The flesh golems are 100% ignorable. The party focus-fire'd on the Lich and he died in the second round of combat before getting a second turn. As fodder, flesh golems represent too little of a threat and the lich too obvious of one. At best, the golems form a wall and block advancement, but they can't actually harm anyone due to low to-hit bonuses and low damage output. [Possible fix: Start the lich with foresight + great invisibility up; replace flesh golems with stone golems]