There was an episode of Basic Brewing video some weeks back where they did some testing on extract efficiency vs. the volume of water used in the mash per pound of grain. They discovered that although 1.25-1.50 quarts per pound is the typical recommendation, they saw a much higher extract efficiency when they mashed with 2 quarts/pound.
Additionally, I had a really low efficiency on a batch I made a couple of months ago. Enough that I ended up adding about 3# of DME to get the gravity up to where I wanted. I went back through my notes and evaluated all my processes etc. and couldn't figure out where I messed up. I finally discovered that the digital thermometer I used on that batch was broken, but not broken such that it was obviously wrong. So, I had done a multi-step mash all at completely wrong temperatures (all too low) and then done my sparge at a super low temp to boot.
Right there are two different factors that have nothing to do with crush that can cause wildly different results in extract efficiency, and I'm sure there are plenty more where that came from. So, when people immediately point to the crush as the source of all their efficiency woes, I am suspicious about that really being the primary cause. It's much more likely a combination of factors than any one thing.