I've played half a dozen games of Agricola — enough to get a feel for it, but not enough to know all of the cards or really weigh their balance.
It's my impression that the Basic and Interactive decks are roughly the same in power, just with (as expected!) more player interaction in the latter.
Ignoring any particular cards that might be outliers, is the Complex deck simply more involved to use, or are the cards in it generally more powerful?
My initial, gut reaction to this question is "no, the Complex deck is not intrinsically more powerful".
Sure, it has some truly standout cards in it - Wet Nurse, I'm looking at you - but for every one of those it has a card with little to no good applications, e.g. Smallholder. There are some strong Minor Improvements, but again, as well as all the excellent cards there are near-last-picks-in-draft like Yoke.
Are the E and I decks weak? I'd say not, not really. Occupations like Clay Mixer or Field Watchman, used well, can be just as powerful as anything in the K-deck. A Plough is pretty much a Plough, and I'm always happy to have access to one, whichever deck it comes out of. Some of my favourite Minor Improvements of all are to be found in the E-deck: Private Forest, or the potentially completely insane Clay Supports.
I often draft all the sets mixed together and I'm pretty sure I've never observed any kind of phenomenon of K-cards being snapped up quickly and E- and I-cards being left till later. If anything, because what you want is cards that have simple positive effects and work well together, too many K-cards with lots of complicated text are liable to be counterproductive. Yes, you'll first pick Wet Nurse if you see her. But you'll spend the rest of the draft trying to pick up straightforward utility Improvements and Occupations that back up the strategy suggested by your first pick. As such, my intuition is that I probably favour Basic and Interactive cards over Complex ones at most stages in a draft.
tl;dr: Complexity can be very powerful, when it does things that are not normally permitted in a game of Agricola. But synergy is almost certainly more important. And it tends to be easier to find synergies between simple cards than involved ones. Therefore I strongly suspect the K-deck of being no more powerful, or just a little bit more powerful, than the others.
It'd be a really interesting experiment to play some games of Agricola between evenly matched players, giving them cards from different decks to play with, though, I must say! Would kind of defeat the main point of the I-deck, mind you...
we have recently played a few games with only K cards - and they actually seem weaker! (with a few exceptions, of course.) gameplay need be more precise to take advantage of the relatively feeble improvements they offer. also, we got the impression of the deck intentionally making it harder for all players by focusing on wood as a prerequisite for nearly all Minor Improvements. therefore, we tend to use the K deck when we feel extra competitive.
