The 5 stars are strong without any constellations. They are probably immortal at 6 constellations but ain't no one got money for that. c6 4* is stronger than c0 5*. Some 4* really need constellations to be useful, some are good without.
For example, my razor is c0 but he's my best character (even better than the 5* I have) but a lot of that is because I have 5* weapon on him. On the flip side of that, I tried ningguang at c0...i couldn't switch her out fast enough, she's so weak at c0.
Anyways, why are you asking this? Are you thinking of putting some money into the game or quitting? You get what you get as f2p, there's nothing you can do about it so don't worry about that.