Mustard oil is rich in unsaturated fatty acids, but it also contains a special type of fatty acid called erucic acid, which lies at the center of the controversy surrounding the oil. Seeds from the brassica family of plants, which includes rapeseed and mustard, in addition to cabbage and kale, all contain varying amounts of erucic acid. Early experimental studies on animals in the 1950s suggested that erucic acid possibly had a role in the development of heart disease.
From: https://www.seriouseats.com/mustard-oil-guide (there's a lot more and it's worth a read)