I've pulled stats of nearly 300k AR45+ players who have used Akasha to answer some questions like how many players are clearing Fearless mode of Stygian Onslaught and how it compares to Spiral Abyss and Imaginarium Theatre.
15% of players have cleared on Fearless mode. Since coop clear times aren't shown on profile, this doesn't mean nearly 20% didn't get primogems rewards from clearing Hard mode. Fearless mode in indeed much harder than Spiral abyss and Imaginarium Theatre.

Here is % of players who have gotten all rewards from respective modes. Abyss is now easiest end game mode we have
>Game mode / Number of players getting all rewards
>Stygian (Fearless) 15%
>Theatre (9+ stars) 26%
>Abyss (36 stars) 40%