I love the reaper autocannon. It gives you more range and does more work as a result. It thins out hordes better. When you play Necrons with quantum shielding you'll appreciate high STR with low damage with more shots.
The math disagrees though. Launchers are more effective vs Cronshields as well, and 8 S7 shots at 36" is not why we run 10-man BL squads.