Why isn't Reaper Shadow Rage used often?

[Elasah]

Besides player preference, but based on many videos Shadow Rage (ult) isn't used as often, it seems really beneficial though

1. Increased action speed for your skills, dish out more attacks
2. All your attacks reduce its cd (it's cd is restored via fire-physical attacks)
3. You can move normally and do whatever while ult is active
4. Your special attack lets you dash extra while active
5. Your regular attack is boosted/transformed into a stronger version

[Milo]

It's used often in some videos. It completely depends on whether the player knows the mechs well enough and how to use them. The reaper ulti can become a clusterfu.ck if you don't know what you're doing, and you'll sometimes have a hard time seeing what the enemy is doing. Also, when you do the reaper ulti, you tend to look at your hotslots to make sure you release all of your skills off cd, and this can be somewhat dangerous if you don't know the mechs.

[Tet]

To add on a bit to Milo, it's a very situational thing.

While [Shadow Rage] does enhance your auto and special click, that can be very odd to deal with. More often than not, my special click is now useless because of [Dash] and the left click is really only good for the damage transference with [Rain of Death].