Contrary to the apparent opinion of several on this blog, the "bust" rate of international players who have been drafted in recent years is not really much different from that of NCAA-sourced players. For lottery picks, the bust rate is about 1 in 3. That increases as you go deeper into the draft for both NCAA and Internationals.
I found some data that contradicts this opinion. It says that 58% of foreign players are busts. 49% of college players are busts and .32% of high school players were busts. 10% is a huge difference in my book.
http://sportsnstats.com/nba-draft-euro-bust/
That article uses data going back to 1979. While it gives a larger sample size, it is also skewed away from the actual trending. It also is including busts from later in the draft in the two numbers you cite.
I did a sample on lottery picks since 2005 and came up with essentially about 1 in 3 busts overall, just slightly above that for non-NCAA players and slightly below it for NCAA players, but both numbers easily within the noise of 1/3. That was posted on one of the threads on these forums.
It is also perfectly reasonable to expect that, historically draft selections of international players would be subject to more uncertainty (due to lack of information) and thus more subject to "bust" not necessarily on the innate lesser ability of a whole class of players, but on improper evaluation of the selectors. The more information you have about a player, the more likely you will correctly rank him relative to his peers.
As we move forward to more recent times and going forward the amount and quality of information available to GMs, scouts and analysts on non-NCAA players has increased tremendously.