I noticed there are a few monsters like this in the MM that go both ways. For example, did you look at the Chasme Demon? Holy mother of...The thing does 64 points of damage a hit with no save. Can you imagine a group of chasme attacking a party? It would make a group of vrocks or higher level demons look like chid's play. Yet they are CR 6. How many 6th level PCs can take a 64 point hit?
Actually, quite a few. The chasme only does 40 points of damage on average. Most 6th level wizards have 32 or 38 hit points. Most 6th level fighters have 52 or 58. It's the 78 points of damage if it criticals that really matters because that will kill many mid to low hit point PCs with a single hit. The hit point maximum reduction probably won't matter that much in a combat (although it might affect future encounters that day), nor will the Drone ability.
But a group of chasme is much higher than a CR 6 encounter and I wouldn't expect a group of 6th level PCs to fight them. One chasme is considered Easy by the CR guidelines for a party of 4 6th level PCs, two chasmes are considered deadly.
Then you have the Demilich. Not sure what they missed on this creature. A level 17 wizard with power word kill gets to the Demilich and kills it. Only way the Demilich lives is to encase the wizard in an antimagic field and keep it on him until everyone else goes or get lucky with the howl ability. If the wizard gets to go, the demilich dies immediately.
I consider this to be a flaw of Power Word Kill, not the Demilich. PWK is an enchantment spell without a save. Although the 3E version of it also did not have a save, it did have spell resistance. In a game where creatures get to save every single round vs. spells, they kind of missed the boat on this one.
Even with the gnoll encounter, if that had been kobolds or hobgoblins half as many would have been too much. The kobolds would have died easier, but all those attacks with advantage even with missile weapons would have done some damage. Hobgoblins are nasty in groups.
It's an odd game. I don't feel like the gnoll encounter was really a CR 8 given their special abilities aren't as good as other special abilities. Even orcs are somewhat scarier because you can't outrun them and their swingy damage can really hurt you. One orc hit is like a gnoll crit. An orc crit is like a hit from four gnolls. It's bad news.
I think this is a relic of the CR mechanic, lack of granularity. For example, wolves are at the very top of the CR 1/4 food chain (I'd usually prefer to fight a zombie than a wolf). Not quite as powerful as most CR 1/2 creatures, but definitely one of the strongest of the CR 1/4 ones.
It's also why I started this thread. My wizard felt like the gnoll, not the orc.
