The disappointment seems to stem from wanting the black dude to be impeccable and win singlehandedly.
The disappointment stems from wanting the ONE black character to be treated equally to his white co-stars.
Here is what
every other Star Wars "lead hero" (not the "mentor" character like Qui Gon, Obi Wan, and Han) accomplished in the climax of their first films :
Anakin got to destroy the droid control ship.
Obi Wan got to defeat Darth Maul.
Luke Skywalker got to blow up the Death Star.
Han Solo got to save Luke's life in the Millenium Falcon.
Rey got to defeat Kylo Ren.
Poe got to blow up Starkiller Base.
Finn got to be put in a coma. After
attempting to defeat Kylo Ren and
failing. No one else fails in their final attempt to become a hero.
I can't believe anyone is satisfied by that and calling it proper "growth" for the character. He "grows" from a storm trooper nobody to a coma patient nobody.
He has earned the pity of Rey for his endearing attempts to rescue her, but the respect of no one.
He has contributed nothing to the final victory because he has no special skills.
He has no discernible future because he can't fly like Poe and Rey and there is no indication that he has the potential to become a jedi.
Rey has inherited the lightsaber, the Falcon, and the Force. She will leave Finn behind to train with Luke Skywalker.
Poe is the best pilot in the Resistance, and will continue to lead the fight against the First Order in his special black X-wing.
Finn... might not wind up a paraplegic? Here's hoping.
It really looks like there is nothing more the character can be used for than to provide humor as a sidekick to Poe or Rey. Without a special skill or distinguishing characteristic of any kind (like how Luke had the Force, Han had the Falcon, and Leia had leadership of the Rebellion) there's really nothing more for him to do.