Omega 3 fatty acids in flax seed (as well as in Hemp Protein) are found in the form of Alpha-Linoleic Acid (ALA). Not only is ALA not sufficient to supplement on its own, but ALA has to be converted by the body into a usable form, and the ratio of conversion from unusable form to usable is rather poor, somewhere in the range of 5-15%. Omega 3 supplements in the form of EPA and DHA are what the body tends to use for many of the benefits associated with Fish Oil.
For vegetarians and vegans, supplementing with DHA from algae can "markedly [enhance] the DHA status (of serum and platelets)" and "[provide] for the formation of substantial EPA". Supplementation of ALA and/or GLA is not enough.