My friend just got a quickfire rapid and tells me his "left-shift" and "right enter" is "mushy" and feels like Grinding..
This occurs when you press down near edges of the key cap.
I checked his stabilizers and keycap alignment, no problems..
I took apart the keyboard, and it seems the "switch itself" is SLIGHTLY by < 2 degrees tilted off the "plate"..
So I desoldered the switch, and resoldered it.. and the mushiness is gone..
The point is,, if the modifier switches are tilted EVEN by a little bit... in this case, by the thickness of a single piece of paper, it can Cause mushiness.
Easy fix for anyone with this affliction, as I've seen post about it before.