The hollow part of it i dont think is really at fault all by itself, Its a combination of having it hollow and having it drilled out to have the pin in the side. Thats the weakest spot of the hub. I actually drilled the hole in the axle deeper and then tapped out the axle to take a longer screw, i then took the pin and cut it in half (i cut it down alittle more than just 2 half pieces) because now the pin wouldnt be able to go through the axle all the way anymore then i just carefully assemble the hub with the two seperate pin pieces,

I never actually thought about this but it might be feasable to just tap the hole where the pin goes too and then just put in 2 set screws to act as pins.

I actually had one break on me like you did when i first got the truck i flipped it over at about half throttle on dirt/grass and the front right wheel flew off. I noticed the metal of the axle looked very pourous and figured it to be defective, so i did the above mentioned mod to the new axle i put in it, but the axle's come in a set of two so i had a spare and i didnt bother doing the other 3 unless another one was to break if it did then i would of replaced that one and modded all 4 axles, but ever since i havent had a problem.

( I actually did this mod to the broken axle while i was waiting for the parts to come in, i just put the two pieces back together and used the long screw to hold it together, it worked fine but eventually i would flip or something and the screw would bend where the axle was cracked, so i would just take it out and put a new screw in and continue on until i finally got the replacements)