There are several issues or bugs in the firmware that have been reported and a few acknowledged. The only way I feel secure using my CDJs so far has been using CDs only. I am not saying they don't work with a HDD or with RB but I have had them freeze, lockup, etc.
I am happy that you have not experienced issues but the easiest way to tell you of the different issues is for you to read everyone's complaints. Some guys have also posted videos as well.
What it comes down to (IMO) is that I did not pay over $4000.00 for two CDJs that were advertised to do so much more and then to be afraid to use their features. It's like saying you buy a super car but being afraid to go fast in it cuz the engine might blow; and to add insult to injury, the company that manufactured this wont do anything about this.