TBF, It's partly their fault. There are at least 2 major fuckups GBX did: 1. not respect the promise 3DR did to it's fanbase of releasing the modding tools (and never issuing an official statement as to why they weren't released, not to mention Randy expressed interest at one point in releasing DNF modding tools prior to DNF's release) and 2. practically the entire Duke fanbase agrees the game needed more polish. Sure, the game's absurd development time and the version GBX got was all on 3DR's fault, but the actual end product that was released to the masses is on GBX. They saw no problems with the game and only slightly modified a few things (namely, Duke's model, the twins and seemingly a few slight cosmetical downgrades. Compare the 2009 Triptych trailer to final DNF to see what I mean) without addressing any of the major complaints about the game (such as the weapon limit, health regen, levels being overly linear and strict, etc)
Again though, it's only partly their fault. 3DR was the major reason DNF's development turned out the way it did, obviously.