Mixed gendered relays aren't really even as far as the importance of male and female athletes (or worse, in the swimming mixed medley relay, men who are good in specific strokes are more important than women who are good in specific strokes, and vice versa). It is especially blatant in the mixed snowboardcross relay, where the female athlete is just way more important than the male athlete, or in the short track mixed relay where the male athletes are way more important than the female athletes.
This kind of strategy doesn't really bother me in the same way in a same gendered relay because there is actual strategy around the order, as opposed to mixed relays where the order is set, and one gender is just inherently more important than the other (or the order isn't set but there is definitively a specifi strategy that is better, like in the mixed medley relay in swimming).
And anyway, relays are meant to demonstrate some amount of depth countries have, and mixed relays just don't do that as well.
As for why most people here dislike the mixed 4x400m relay: it's redundent with the same gender relays that already exist, and the primary country in 400m running ( ) just doesn't think it matters enough, even at the Olympics, to take it seriously.