I don't think there's a power creep issue with Adventures.
Nothing pushes for power creep in Dominion - it won't sell sets, it's not good for the game. Designing cards doesn't entail fighting it. It's something with no reason to have it and I'm not trying to. So there's no reason to expect it and I don't see it.
What power level should cards be? This depends on how fast you want the game to be, and how much luck you want it to have. The more powerful the cards are, the faster the game and the more luck. I aim for a power level that gives a game length and luck amount I like.
Once you've established a good power level, ideally all cards are close to it. If a card is too weak, you don't play with it; if it's too strong, you don't play with other cards.
There are some caveats:
- It's nice once in a while to have an extra-narrow card, so that you can have the joy of winning with a card you usually can't win with. Originally I thought maybe 1 in 25 cards; now I think more like 1 in 100.
- Some cards only compete within their category, or only with certain categories. A strong trasher makes you not buy the weaker trasher, and may shut out a weak card that pushes a non-trashing strategy, but may still leave you interested in a weak card-drawer (that's as good as the table offers).
- Some categories are historically imperfect, in a way that I can't completely fix. I can make new cards weaker but don't want to make new cards that can never compete in those categories.
- Cards can't have identical power levels; something will be the strongest in its category.
The main set and Intrigue both have more duds than later sets; Seaside has more duds than later sets, but not as many as those two. Power creep isn't about the weakest cards though, it's about the strongest cards. These sets have some of the strongest cards in the game.
Events do not have the same power level issues as kingdom cards. They affect game length and luck, and how good the 10 cards are, but strong Events can still leave everything playable. Like, maybe it feels like Alms must be too strong, since you're likely to buy it at some point in any game with it. But it's entirely dependent on the 10 cards; it spices up the game while letting you play with whatever showed up. Ultimately it lets us have a fun game of Dominion, and it's okay that it improves things for everyone. It nudges the game length and luck level but not at the expense of kingdom cards.
So then, when I look through Adventures, I see all these sweet cards, cards which I expect to see a fair amount of play, which are not duds, but which are not the tops in their categories. Swamp Hag isn't the strongest Witch and Ratcatcher isn't the best trasher and so on. If we randomly divided cards up we would expect Adventures to have a few of the strongest cards, and maybe it does, but it's no outlier.