Well they don't generally they're more mediocre imho. But why? We know WotC is capable of making good ones.
The big problem is the format. It's very hard to design a level 1-10 adventure let alone 1-20. It's an art form not a science.
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I agree wholeheartedly. I wouldn't say they suck but I don't enjoy the fact that they are effectively whole campaigns (not counting here those couple of books which are collections of shorter adventures).
Roughly (not strictly) they assume/require to start with new characters, level 1 or slightly higher, and at the end you are so high level that you won't find another adventure to continue with them, so unless you homebrew the continuation AND actually want to run a tier 3/4 game, you retire those PCs and play the next published adventure with new ones.
The positive aspect of this, is that you wade through the character material: if you played one official adventure per year during 5e, by now you have probably experienced almost all classes at al levels.
On the other hand, it has several aspects I don't like:
- the narrative doesn't vibe with me at all: my ideal narrative is that an adventurer (PC) lives through A LOT of adventures in a lifetime, not just THE ONE adventure of their life: more Indiana Joneses and less Frodos for me
- narratively again, I don't want to always have to start with an apprentice, who then breezes through levels and becomes a legend... in a week of in-game time
- I actually dislike having to control my PCs level advancement to stay up with the adventure challenges
The last point really bothers me as a DM. I don't like this adventure format because it clearly has an artificial difficulty progression: you're supposed to reach level X by the time you are in chapter N, well NO THANKS! There's a tolerance for sure, it's not like the game breaks if you're a couple of levels off, but to me it is pretty much the idea that you have to bump up the level to match the story which I dislike. And if you do that properly, the adventure difficulty doesn't actually increase.
My ideal adventure instead is one during which nobody levels up. You play the whole adventure at the starting level. So when you reach the BBEG that part is normally actually harder than more or less anything in the adventure so far. When you play at fixed level, players can stop worrying about XP and focus on in-game power ups (treasure, knowledge, social gains, equipment...) and for instance there is no shenanigans like XP farming from useless critters. Then at the end of the adventure, you count the XPs and see if you gain a level (and if you want, add a narrative on learning new abilities off-screen). I know this is a bit extreme but works for me, and I don't have usually problems doing so with an adventure that has for example a designed 3-4 levels spans: I start it when then PCs are 1 level lower than the level they are supposed to reach before the final part, and keep them there: so the whole adventure plays easy-to-hard instead of mostly the same. But I can't do that if the adventure has a 10-15 level span!