Usually nothing. Reshoots are like anything else and are as capable of being bad or good.
I would be concerned if James Cameron were forced to reshoot an ending, or Martin Scorsese, or Quintin Tarantino, or any number of established directors that know how to make movies. But this is Simon Kinberg, a guy who has no experience directing films but has experience writing and producing a bunch of SHIT in spades. Reshoots are probably a blessing here but I doubt they will save the film.