While Infinity pistols are dropped from a boss a decent distance from any spawn point, and said boss sometimes throws his loot to accessible areas, Shredifiers are not tied to any particular boss. Making them rarer, and of course- inferior to the Infinity.
I have yet to get a shredifier, so I wouldn't be able to argue. Who does it drop off of? I have heard it drops off bone head, but then I have heard people say it drops off random enemies
Worst legendary would have to be "the cradle" or "Logan's gun." No matter how horrible shredifier may be, I cannot imagine it being worse than those two.
It's easily more useless than Logan's Gun. I can kill Hyperius with no bee shield and a redundant logan's gun fairly easily- so long as I can use a corrosive badaboom on his minions in concert with a fire badaboom for when he starts jumping around. Also, when using an absorption shield, the logan's gun can regenerate your rocket ammo, which is useful if you play no bee. :o