Like the only downside to lucio I can think about is that he doesn't do as much damage as the other attacking classes, but he still does a decent enough damage to wear down on enemies, doesn't actually have to decide between attack or healing, has his knockback which can kill people in certain maps, and it's not even his main purpose.
He has better survivability than the other supports and even quite a number of the assualts/defense heroes, insane mobility, basically is a HP+ for a group just by being near them, and his ult is only second to mercy's when it comes to pushing/surviving a push.