There's a really popular and really poor article floating around the internet (it's a virtual flood if you try to do searches on Canola) (example: http://www.westonaprice.org/health-topics/the-great-con-ola/ )
It comes from a Washington DC anti-GMO "think tank" currently receiving $1 to $2 million a year, about half from member fees and the other half (i believe) from hosting "green" conferences. They state most of their membership is from small farmers, probably fighting against GMOs. The article was written in 2002. The article was written by the think tank's founder and co-founder with degrees in english and nutrition. Both the think tank and the writers have criticisms written about them in Wikipedia. This is probably the original source of 95% of all negative comments against canola oil, which has more omega 3s than any other popular oil. Flaxseed is one of the very few that is better. Olive oil, grapeseed oil, corn oil, peanut oil, etc (you name it) are not high enough in omega 3s. None of them meet the 4:1 advice of many nutritionists, which is where Canola comes in. Soybean oil is actually a lot closer than most. Canola is also not more prone to rancidity than the others, and I think a lot less than flaxseed.
The anti-canola article is mostly "side-swipes" that are not related to human health. You can find the article by searching for "The Great Con-ola".
References 17 and 18 in the article argue against canola for human consumption. But these studies SUPPORT human consumption of canola. The studies were in rats bred to be hypertensive who needed cholesterol to mitigate the rigid membranes they were born with. Canola's cholesterol-LOWERING ability was why the mutated rats did not live as long and point out "OLIVE oil-fed rats had the lowest survival rates despite a very low phytosterol concentration" and conclude "Thus, canola oil, because of its low concentration of saturated fatty acids and high phytosterol concentration, could be considered more beneficial than other common dietary fats in lowering the blood lipid risk factors for the development of CHD." Now contrast this with what the errant anti-canola anti-GMO think tank's article states: "[sterols in canola] make the cell membrane more rigid and contribute to the shortened life-span of the animals".
Concerning reference 12 supporting the statement "high levels of omega-3 fatty acids correlated with high levels of lesions." The reference said diets without ANY saturated fat are harmful to rats. Soybean oil and canola oil-ONLY diets were both harmful, not merely that "high omega 3s" are harmful. Likewise for references 13, 14, 15, and 16: diets containing ONLY canola oil without saturated fat or excess vitamin E were harmful in animal testing. This is an important distinction from saying "high levels of omega-3 fatty acids correlated with high levels of lesions", "piglets fed canola oil suffered from a decrease in platelet count and an increase in platelet size", and "Canola oil was found to suppress the normal developmental increase in platelet count". These are all true statements, but there is crucial context missing from the original research.
The only important and dangerous-sounding aspects of these papers was the "excess" vitamin E needed to protect against the higher level of peroxidation caused by canola when compared to soybean oil (and olive oil in a 2009 paper).
I looked into this further and found out the filtering process can remove pre-existing Vit E found naturally in the oil which adds to any excess oxidation caused in refining. From a paper: "By development of auto-oxidation [peroxidation] of unsaturated lipids, hydroperoxides decomposed into aldehyde and carbonyl products." It will not taste as well, so there is a motivation to prevent it. If a surfactant like SDS (Sodium dodecyl sulfate), the oxidation greatly increases.
I do not think this increased "rancidity" of canola coming from the refiner should be considered a large drawback because I think it is a direct result of the omega-3s being easier to oxidize, and why they are so hard to find in anything except flaxseed oil and fish oil. And what percent of those are oxidized? I will take an excess of vit E to offset it, if I don't use another source of omega-3's that are less likely to oxidize, like fish oil.
Articles discussing canola oxidation:
"The oxidative stability of refined, bleached and deodorized canola and soybean oils was evaluated over a 30-day dark storage period at 65 °C. Peroxide value (PV), conjugated diene (CD) and triene (CT) contents, 2-thiobarbituric acid reactive substances (TBARS) and p-anisidine values were determined. In addition, NMR spectroscopy was used to monitor relative changes in the proton absorption pattern of the fatty acids of oils during storage. Canola oil showed higher PV, CD, CT and TBARS as compared with those for soybean oil. The ratio of aliphatic to olefinic protons in both oils, determined by NMR spectroscopy, increased steadily over the entire length of the storage period, indicating progressive oxidation of unsaturated fatty acids in both oils."
"The content of primary (PO) and secondary (SO) oxidation products of canola oil was measured, and a good correlation obtained between them (R2 = 0.991). Peroxide value showed the highest contribution (53.69%) to the PO, whereas its contribution to the SO was much less significant (0.36%). Acid value contribution to the production of hydroperoxides and carbonyls was 12.75% and 29.82%, respectively. Polyene index showed a relatively low contribution to the PO (6.71%) but contributed highly to the SO (21.82%). Tocopherols were more effective to prevent the production of hydroperoxides (14.77%) than of carbonyls (4.36%). In contrast, phenolics were found to be better than tocopherols to resist off-flavours production (11.64% vs. 3.36%). Total polar compounds had a pronounced contribution to creation of off-flavours (32.00%) as well as a marked effect on the PO (8.72%)."
a full paper on it: http://www.sciencedirect.com/science/article/pii/S2211601X11000162
Reference 19 (Federal Register, 1985) was supposed to be the support for the statement "it seems to retard growth, which is why the FDA does not allow the use of canola oil in infant formula". I could not find that regulation, but it was cited in one of the references, giving a different reason: "The use of canola oil in infant formulas is not permitted because infants fed formula might consume higher amounts of 22:1(n-9) than would be provided in usual mixed diets and because of the lack of data about infants fed diets containing canola oil. (Federal Register, 1985)"
Someone finally applied for GRAS status and has the green light to be used in infant formula in preference to soybean oil as in other countries:
These are all the references that were health-related to canola oil. The rest of the article has a lot of unorthodox theories concerning types of fats.
There was also an anti-canola spam email that snopes replied to: http://www.snopes.com/medical/toxins/canola.asp