I'm not sure how true they are but a couple reasons I've heard for the 383 spun bearing condition are the following:

1) The factory rod bolts would stretch on the high revs and were thus the weak link.

2) The 383 at high revs would suck the factory oil pan dry, even with the stock volume oil pump and cause momentary oil starvation at the rod journals.

Again, don't know if this is nothing more than hearsay, and certainly these issues could manifest in any stock engine that's over-rev'd. That said, the 383 was mass produced and market towards the value crowd who undoubtedly ran the holy hell out of these engines. The notoriety of spun bearings and blown blocks, probably a combination thereof...

In any case, the two conditions mentioned above; pretty easy to remedy.