Finished the campaign at level 13. I'll try to throw together a concise Steam review sometime in the next few days. I'll note that I got nowhere near enough cash for all building upgrades, and even my best weapons were no more than half upgraded. Other than the boss fights, though, I was totally dominant for the last third of the game. I played on "Vigilante" difficulty, but in retrospect I wish I'd started on "Hard Boiled." Couldn't have known that beforehand, though.
Once you get to the point of being able to knock one boss around, you'll probably have the intel to do the other two in short order. It's almost a shame, I was really enjoying the ridiculous SMG the cultist leader drops. I know it would have been out of scope, but it would have been awesome if after taking out the first boss the game threw some kind of curveball showing the other factions' desperation. Events like joining forces, calling upon corrupt cops, or even hiring an on-call hit squad that sometimes shows up during regular missions.
I did notice that the sound for the fountain on the Mafia boss map is bugged. It always plays at full volume, even if sound effects are muted.
For games I've Kickstarted, chalk this one up under "no regrets."