I guess Alfie Kohn is someone I should read up on too - he has some interesting ideas, which I should probably try to understand beyond the sound-byte level. The criticism section on the WikEd is a bit better furnished than on Wikipedia - apparently some blame him for damaging the US school system.
Ah, this article provides some criticism, with some pointers to research. Alfie Kohn put up a rebuttal that seems to address the points pretty well. The existence of criticism doesn't say that much, I'd expect some for any successful writer, especially in a somewhat politicized field like education. I also found this by J. Martin Rochester, unfortunately most is behind a paywall.
I also stumbled upon this criticism of constructivism, and some notes and criticism on the paper. Judging constructivism as a whole may be a bit too much for me though. I'll probably need at least a rough understanding of what the competing points of view in education are, I'm still far from that ...
I found this interview interesting, but it seems that the split between constructivists and traditionalists has a bit too much to do with US politics (or at least, the politics of education). Trying to figure out who's right and who's wrong probably isn't the best use of my time, I should focus on specific issues instead. Knowing about the big picture can still be useful, but mostly for putting more context around ideas.