Waiting Gears 5 (standard edition) release day, I recently re-installed and played Gears 4, and at the same time I finished Man of Medan, both games played in HDR using option 1 with DC medium.
It is absurd how the two games look different in HDR: Man of Medan is extremely dark, with horrible grays instead of blacks, at the point that i found difficult to actually see entire scenes and to finish the game. On the other hand, Gears 4 looks superb and super bright, even with DC set to medium, and offer an actual HDR luminance slider that really works.
This demonstrates that, aside from tv HDR brightness (or not), a big difference is found in HDR implementation. If every game looked like Gears 4 (and Gears 5, I hope), we probably wouldn't even talk about dim HDR game mode.