What makes ESG unique is the degree of disagreement regarding what should go into an ESG score and how those metrics should be weighted. Further, there are no simple ESG definitions, no book-to-price equivalent of ESG that can be applied universally.
We are further limited by lack of history and standardized data. To quantify ESG, we need comparable data across a wide universe of stocks so we can rank companies consistently. The good news is that in addition to third-party scoring systems, efforts are underway at organizations such as the Sustainability Accounting Standards Board to create standardized reporting; but even with standardized data metrics, subjective judgment will always be a part of ESG definitions.
There are several organizations that have ESG models whose output can be thought of as ESG factor proxies. For example, MSCI has ESG Ratings, S&P Dow Jones has DJSI scores and FTSE Russell has its FTSE4GOOD scores, to name a few. Each of these has different universe coverage and history. Looking at these data sets, we can better understand the risk-and-return dynamics of ESG, definitional differences notwithstanding.