Agree completely on cost of ownership, parts avialability, and the ability to customize any CUCV is significantly more favourable. However, also consider that some things you would do to customize a CUCV have already been done by default on a G-Wagen. For example full 100% locking differentials or coil-sprung long-travel suspension.
L1A1, if you have a Land-Rover Lightweight now, the G-will be comparable handling off-road, thought the lockers will give it a significant advantage. But the G is heavier and larger. It may not look it at first, but it is. Also, the G aesthetics and creature comforts will be completely foreign to a series Land-Rover owner. However, as a long time owner of both vehicles, I can say with confidence that if you put lockers in the Lightweight Land-Rover, you may just find it surprisingly superior to the G-wagen off the road. In my opinion, this is due virtually entirely due to the lighter weight and smaller size. I know I may be thrown in the Stalag for saying that, but hey, that's my experience.
Although I may have already said it, the real drawbacks to the G-wagen is cost. Cost up front, and spare parts, will be like 100% more than a Land-Rover, and probably many times that of a CUCV. But, you do get what you pay for. In my opinion, the quality of design, materials and production QC of any mechanical G-Wagen component is head and shoulders above the rest of the world's trucks, for comparable sized components. Now, having said that, it's also worth considering that any CUCV, and most Long-wheelbase Land-Rovers will have Dana 60 class axles, and no standard G-wagen axle is that tough.
Notice that I previously singled out German mechanical components as being superior, and not electrical. In my opinion, a large number of German electrical components pretty much suck. Just a slight notch above Lucas, maybe... For example, if I haven't driven my Unimog in a few weeks, I have to "Twirl" all the fuses in the fuse box to make thier little tiny pointy ends cut through the oxide and conduct again. Thankfully, the Unimog motor runs without electricity. Now the G-wagen... Well I've had to fix the main electrical distribution panel becuase the feed from the battery fell off, i've had to fix the instrument cluster twice, the head light switch once, the turn signals once, then windshield wipers, and did I mention the stock headlights are lousy? And, since mine's a G500, well there's the the electric windows, the electric sunroof, every freaking rear light on the truck (not to mention the melted and very expensive composite rear light lenses), oh, and you will have to remove something called the K40 relay module and re-solder most of the connections on the PC board about once every 50K miles, and the same for the central locking/alarm module. Then on the engine I've had to replace virtually every sensor on it, and there are many... including one after it left me stranded, and then the o2 sensors (there are 4 of them by the way...). Do you see a pattern developing?
So while I just love my G-Wagen, (which is of true Military ancestry, unlike a CUCV), I could never trust my G500 in the same way that I can trust (and repair) the Land-Rover, Unimog or Dodge in the outback. Though I imagine an older mechanically injected Diesel or Carberated G-Wagen would likely be much more trustworthy. But then the other hand, a Land-Rover is infinitely more appropriate for any L1A1,be it BSA, Enfield, Fazakerly, Lithgow, Longbranch, or god forbid, an RFI.
Again, just my thoughts.