I've been cycling various Rivens, and have found that weapons tend to struggle without a 'perfect roll'. In Dev Workshop: New Mods Part 1! it was mentioned that 'perfect rolls' are not the goal behind the system.
I've put together a video showcasing the performance of 8 Riven modified weapons. Rivens and builds: http://imgur.com/a/jK6oJ
The Buzlok is turned into a beastly crit weapon, in a similar fashion to the Soma (I haven't Forma'd the Soma/S