AD9952 Readback function

Hello,

I'm DFAE. I have a question about the readback function of AD9952 from my customer. 

According to the datasheet of AD9952 and FAQ in ADI WEB site, the readback function of AD9952 seems to use for only a debug or a diagnostic tool. Is there a possibility of error or something issue in the results of the readback? Why is not the readback function guaranteed for a function of AD9522?

Best regards,

Akira