Its ESS's marketing policy not to release datasheets publicly. Seems to work for them, after all they've got a fair few design-ins. Myself - I avoid chips where I can't get the DS without jumping through a series of hoops, there's so much choice out there.
If you don't need 8 channels then the AK ICs look great! I'd like to try those. If you need multichannel I found it more cost effective to stick with the ESS chip.
One has to give the Asahi Kasei engineers credit for making all their data freely available for the most part. Analog Devices, Maxim, TI all make their data freely available. At some point shouldn't you be proud of it and not force an NDA?
