Oh man. OK so here's sort of how it works:
You can't compare stats across weapon types. A low impact Hand Cannon will still do more damage per bullet than the highest impact Auto Rifle for example. In order to determine the power/rate of fire within the same weapon class, however, the impact/RoF bars are sort of a good indicator. I say sort of because even that isn't perfect - comparing Red Death to other Pulse Rifles with different impact and looking at the attack value tells you it ain't a perfect fit. In general though it'll work.
So basically most weapon types will have different "archetypes", or variations on the same template, with some outliers to mix it up. There are low impact, high RoF Sniper Rifles (like everything being sold right now by vendors), high impact low RoF (like Black Hammer, the Iron Banner Sniper, Ice Breaker), and others who fall somewhere in between. There's high impact Auto Rifles (Suros, Vanquisher style) and pea-shooters (Atheon's Epilogue, Dr. Nope etc).
These weapons will play very differently but not be necessarily worse or better. A low impact sniper will fire more quickly, which is excellent for PvP or to chain crits in PvE. It's all a tradeoff and will ultimately come down to your preferences.
