User blog comment:Xanneer/PHYSICS!/@comment-24911350-20140529232559/@comment-24911350-20140530140719

The operative aspect of her flare-like shells are their range, not recoil. The recoil as always been inconsistent to a limited extent, but that force is many times greater with something like a shockwave, precisely because it expands in all possible directions, and firearms don't. Maybe Yang's shotguns can knock someone back a couple feet, but the force that does that is acting on a small part of a single person. If she could throw 4 or 5 or 12 people the same way with with a single shot, that means the shot is carrying a lot more force, which means more recoil.

It's the difference between shooting one cannonball and 12 all at once. Even with a huge margin of error for RWBY-ness, it's really disproportionate.