Sagat’s and Blanka’s low fierces got this (mostly deserved) reputation as being the unstoppable tactics of the early days of Capcom vs SNK 2 play. After a while, Roll-Canceling was discovered which was basically a way to cancel the startup of a Roll into a special move, so that the special move inherited the invincibility from the Roll. So low fierce and RC are the two big scary things that got associated with CvS2.
Every game has them. For Marvel vs Capcom 2 it’s Cable shooting people with the Ghostbusters gun and Sentinel stepping all over them. For Marvel vs Capcom 1 it’s Double War Machine Duos. For Street Fighter Alpha 2 it’s Valle CC’s and Alpha Counters. For Super Street Fighter 2 Turbo it’s Sagat throwing Tigers like a machine gun and Balrog going “RAH RAH RAH RAH RAH!!” (nonstop crazy fast rush punches). For Street Fighter 2 Champion Edition it was “Thud, thud, thud, thud, thud” (the sound of Bison’s Psycho Crusher doing 5 freakin hits of block damage).
By the way, i found a way to Roll Cancel low fierce, but it’s totally impossible to do in a real match because it requires such an elaborate setup. Anyway, just see here: http://lowfierce.com (click on the picture for video clip).