There's a lot I can say here desu. To explain in a little more detail, there are a lot of really good common units, such as Medea, Mash, Cu, and Cursed Arm. But even the "bad" units like Serenity or Spartacus can be really good if in the right teams. There are also some really niche units, such as Mozart who guarantees critical hits for one turn, or Hohenheim who has one of the strongest blue buffs in the game.
Additionally, there's a ton of really good 4-star units, such as Hercules who has both god-tier attack, or Marie who's borderline unkillable due to her three-hit invincibility and healing potential. In fact, Hercules is so damn strong some people will delete their account and make a new one if they don't get him in the tutorial roll.
So there's no need for the big shots really, but there are some fights in the story mode and the Festival events that will make you rip your hair out. The Big Four will make your life significantly easier when you do the hard shit, but otherwise really aren't necessary.