It's not so much that Mu's gatling is bad, but you can't cram all that stuff in with barrier guard pushing you away, at which point you have to board the train to gimmick town and start pushing that C... Not to mention, prior to CP, her 6b was so safe that as long as you first condition people to not mash, there was no reason to not push it.
Also, Mu's anti-jump strings tend to be risky in my experience. 6a lacks the range and 2c is slow and doesn't leave many options if blocked.
Though, who really needs anti-jumpout strings when people just run into 5c in all distances =D