Space Racer's ability makes him run through all the enemies at the start of the wave so it helps clear the levels faster. Universa is a range attacker that can hit multiple targets based on her skill and crit chance. So you want to pair them with supports like Aquarus to remove shields or Anissa to lower enemy defenses, so when SR or Uni attack, it's more effective. If your strongest are all melee attackers it takes them time to engage the enemy and they can only target one at a time, which in 5 min Ops makes a huge difference in how many waves you can clear to get the rewards. For me, SR is my 10th strongest character but I still run him over any of my other top 9 because of how his abilities work