Both methods can produce excellent extracts. CO2 is a good extraction method. CO2 processing can be adjusted to several temperature and heat parameters that are selective for cannabinoids, terpenes, and many other compounds that are found in the hemp feedstock. CO2 processing by itself makes a crude oil in the first run that attempts to maximize the yield. The crude oil contains the chemistry of interest and many other (100’s) of compounds. In order to clean up the crude extract, it is mixed with ethanol, chilled, then filtered and the remaining ethanol is distilled off leaving a much cleaner and visibly translucent product. This product can then be distilled under high vacuum to get a crystal clear high purity oil.
Other methods using CO2 use co-solvents (ethanol is one) that get better extractions quicker, but still not as quick as ethanol by itself. CO2 can destroy the natural compounds in the hemp—if put under too high a pressure and heat in the acidic CO2 process. The results under high pressures and temperature can be ethanol esters of the plant oils and other compounds. Besides that, CO2 by itself is reactive and acidic in nature. Acidic conditions and pressure are more likely to create side reactants from natural compounds and acetic acid from any water present.
Ethanol extracts are much more straightforward. They can be done at hot or cold temperatures, run under vacuum, atmospheric pressure, or slight pressure. The extractions can be done super cold or room temperature and up to the boiling point of the ethanol (although cold-room temp is better). Mild extraction conditions are gentler on the compounds. Ethanol dissolves the cannabinoids faster than CO2 also so more cannabinoids are retained in a shorter period of time. This means the cannabinoids are more soluble in ethanol than CO2. Extreme pressure and heat are used to make the cannabinoids soluble in the CO2 where ethanol dissolves cannabinoids at normal temp and pressure. It takes a liter of liquid CO2 to dissolve 0.2 grams of cannabinoids. One liter of ethanol can easily dissolve 200X – 1000X the same cannabinoids and at milder extraction conditions.
Ethanol also extracts plant pigments along with the cannabinoids if the conditions are warm enough. Similar to the crude extract from CO2, the extract needs to be cleaned up by winterizing then by high vacuum distillation if a crystal clear product is desired.
Either one of these is a good method. Both use ethanol as a food-grade solvent if the extract will be cleaned up and turned into clear oil. Either one can produce extracts over 70% total cannabinoids in the first run.
When you use CO2 you still need to use ethanol to purify the extract in the winterization stage. CO2 extracts can separate out different fractions (but not very good on common machines) and can keep more terpenes in the extract before winterization. After winterization and decarboxylation the extracts will both have lost some of their original terpenes.
This is a very broad topic with lots of opinions on either side. Both methods work very well when done right and any possible residual solvent is not only GRAS (FDA classification meaning “Generally Regarded As Safe”) but not a petroleum product.