There are zero technical challenges to putting user-made mods on consoles except the DRM that Microsoft and Sony have put in place for the specific purpose of keeping user-made mods off the console. From the technical perspective, there is not the slightest difference between a user-made mod and an official DLC or expansion like Broken Steel or Shivering Isles. It's simply a business/legal decision. I'm surprised this hasn't been locked already, since all the companies involved seem to regard end-user knowledge of these issues to be thoughtcrime.
Bethesda had to push back the release of the FO3 DLC on the PS3 several times so there are apparently some technical challenges.
This doesn't mean that getting a mod to run is difficult, it probably means that getting
multiple mods to run is difficult. And in fact this is the reason Bethesda gave: they were having trouble getting the PS3 to handle running all five DLC at once. If you look in the hardware forums for the two consoles (moreso the PS3) you can find a lot of people encountering issues that seem to stem from memory problems. The PC can encounter these problems as well but most people playing on the PC either have lots of RAM or are playing on reduced settings, and you can tweak the PC version down more than the XBox or PS versions.
On the subject of the PC running mods, some mods are real resource hogs even on PCs, and some don't run well unless you have a really high end system (and honestly some don't run well on any system

). This wouldn't be improved by releasing those mods on a console even if it was allowed. As PS points out some mods can be extremely large, larger than even the official DLC. If Bethesda is having a difficult time getting their mods to run smoothly on a console I don't see the situation being any better for these large user-made mods.
So while the technical challenges of getting a mod to actually run on a console may be slight, the technical challenges of getting a mod to run well - or of getting many mods to run well at the same time, is far from easy.