So I'm thinking this syn from recent event may be a bit underrated.
It gives you -10% Accuracy and Recoil to start which is as good as Run-n-Gun.
Then you get +10% Damage to shields and it seems more enemies have shields these days so thats a nice boost.
+10% Crit Multiplier is better than you can get from Assassin syn.
And finally the biggie, Causes enemy shields to explode and electrocute nearby enemies when broken.
This last bonus is where I'm really seeing this synergy shine as it can wipe out whole mobs at one time.
Go to Silicon Valley and watch 99rs, Chosen, and Hellbugs all get flash wiped in bunches. Hit an Afflicted Siege and start clearing entire waves at a go.
Previously my OJ Radient Solstice GL-1 Ground Pounder was my weapon of choice for mob clearance gun is a beast.
Considering the Sovereign Grenade launcher drops only a single blast compared to the triple blast of the GP I wasn't expecting it to hold up well in comparison.
However that exploding shields thing can give the the Sovereign Det a much higher effect radius than its listed blast size.
In fact it can reach truly staggering size in large mobs where it goes into chain reaction, so much so that it can become a threat to your own safety.
In a Chinatown siege I dropped a 5 round cluster of grenades ontop of the Vaccine Synthesizer in street heading up the steep hill. I was on far side of intersection with this huge mob starting to move my direction. The initial blast cleared the Vaccine Synthesizer nicely then came the chain reaction flash that killed 2/3 of afflicted between me and synthesizer before completely blowing out my shields, scrubbing off two plates of my armor (yay for bio det

) taking half my health and leaving me with poop shoes.
As a mob remover this Synergy may not have an equal in the game, just be cautious not to get to close to your targets.