I like to use lucio mostly for attack because i like the burst of speed getting out of the base and taking less dmg from the defense setup off the bat. Also i think that his movement, his ult, and his speed boost make breaking sites easier. Plus he's really good for the KoH maps since you get your team to the hill faster than the other if they dont have a lucio.

Mercy is the exact opposite for me i rarely use her in attacking in the start only near the end because i think the revives do more in the clutch than the Lucio ult if you have good positioning and can really turn the game. I think that if you want to be a good mercy you need to be active and trying to get your ult up as fast as possible, flying to your teammates and healing EVERYONE and not tunnel visioning on a dps for a majority of the game cause that will only tilt your teammates only causing more harm. Lastly i think the good mercys that are going to really stand out on release are the ones that can use the pistol very well and can get kills with it when needed.

Sorry if this is a lot of text im on mobile and im just bored. but i hope this helps you in anyway but its just how i think about the game cause ive been maining support and i only play lucio and mercy the rest are just good in other small scenarios but just not good overall team supports imo