I got some ESS "data sheets" from Charley Hansen once. Absolutely terrible. I assume it was the best he had access to.
In my experience, the data sheet quality ranking goes something like this:
1. Linear Technology (now ADI)
2. Analog Devices (ADI)
3. Texas Instruments
4. Maxim, NXP, ON Semi, etc
5. Single-market outfits like AKM
426. Secretive companies like ESS
The top 3 are all very good. At level 4 the quality varies greatly between parts, but there's always enough information to get the job done. At level 5 enough information is available to make the device work, but important details may be missing or hard to find. Below that, you're essentially guessing.