Flax provides a form of n-3 fatty acid that is 18 carbons long and out body needs varieties that are 22-24 carbons long. Our enzyme systems are very inefficient at this process so even if one takes a lot of flax oil it does not make much EPA or DHA (the stuff in fish oil and the fatty acids we WANT).
Part of why we want n-3 fatty acids if for the anti-inflammatory effects...well, supplementing with flax oil actually increases the amounts of inflammatory cytokines that we are trying to suppress! One step forward, two steps back. Flax oil will prevent a vegan form keeling over from fatty acid deficiency but it will not optimize performance, health or longevity.