The no round system makes the game favor zoners and vortex characters by ALOT.
A vortex character could just get 4 guesses in a row after they get started and BOOM you're on your second life bar. In any other game with vortex characters (like SF4 Ibuki, Akuma, and Seth), if you get perfected by a vortex its okay because its fair game second round, but not Injustice. Imagine if Seth perfects someone in the first round with his vortex and it was basically a free win of the entire match because he'd be at 100% on his first bar the start of round2....
Zoners are all about the life lead. In other games, just like the vortex characters, if the zoner has a life lead he could just run the entire match and just create an obstacle course for the opponent to get through essentially wasting time and gathering up chip damage. Once the round is over though, its fair game next round. But again, in Injustice if you lose that first round badly the zoner still has no reason to be in arm's length of his opponent.
It's the only reason that characters like Wonderwoman and Flash, who if you breakdown their movesets just appear ridiculously amazing in every way, have a hard time actually winning in this game. They are incredible, yea, but they can't get the almost free wins that vortex and zoners can once they get the the opponent's first life bar down quickly. They have to work just as hard for their wins from start to finish.