The difference between a flowchart and a UX flow is that the first is drawn from the point of view of the game, while the second is from the point of view of the players.
After writing a brief for a new mechanic or feature, specifying everything in a flowchart helps resolve edge cases. Useful before going on to detail the configurations necessary to unlock the programmers.
After designing UI wireframes, a UX flow helps to find missing pieces. Very useful for going on to detail the graphic assets needed to unlock the artists.
If we don’t have time and we need to be quick, the flowchart is the least essential of the two.