Is the Complex deck in Agricola strictly better?
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...