I tend to see more bearing failures on cushion tire trucks.
The 'suspension' provided by the pnuematic tires saves the bearings from much of the shock-loading.

There are many installation problems that can cause premature bearing failure;
too loose
too tight
spindle damage
not enough grease
too much grease
wrong grease
etc...

I believe in installing the bearings a little on the tight side. My logic being that the wheels never do more than about 20-30 rpm, so heat is not much of a factor, and it will help prevent brinelling by distributing the load around the entire race.

I have better luck with moly-h lube as opposed to ep2.

When you examine the failed bearings, what do you see? Is the grease burnt? (ie. stinky) Race failure or roller?