AD9707 AVDD/DVDD/CVDD supply voltages

Looking at the forum for hints on an issue I am having with a design, I found this message: 

I am troubleshooting a board where AD9707 is used and the designer supplied AVDD wit 3.3V and DVDD/CVDD with 1.8V. The data sheet states on page 30 " The analog and digital sections of the AD9704/AD9705/AD9706/AD9707 have separate power supply inputs (AVDD and DVDD) that can operate independently over a 1.7 V to 3.6 V range."

I suppose the designer used different supply voltages based on this statement (the D/A is driven by an FPGA and the IO bus is supplied with 1.8V.)

Now I do not dispute the statement in the aforementioned message (about the supply voltages having to be equal) and the data sheet, despite the statement quoted above, does specify performance only for all voltages equal. But I will need to justify changes I make to the design so I would like to know what the impact of using different voltages of the AD (reliability, durability, dynamic performance, output noise etc.)

Thanks in advance for your support.