The problem with trying to evaluation Nic's effectiveness is that we rarely know what he's really trying to accomplish, either in the short term or in his two-thousand-year goal. (Is he trying to remove Lucifer, because the archangel was nemfected? Is God, like Mab with Maeve, unwilling to kill his most-beloved archangel, endangering Reality itself? Or is the nemfected still in Heaven -- was that why Lucifer rebelled? Or is Nic just crazy -- is his greater purpose a lie that Anduriel has sold him?)
We know that in Death Masks, he was testing out a way to use a holy faith object to power an evil curse -- and his test was a success. The fact that he didn't actually cause mass casualties with the curse, and that Harry took back the (fake) shroud doesn't change that -- those were secondary objectives. (As were recruiting Harry and killing Shiro.)
That test was a success, and in Skin Game, he got away with a much, much more powerful holy faith object. What evil can he power up with the Grail? And why would he want to -- what makes it worth the sacrifice of his greatest supporter?
Small Favor is even stranger -- the whole book is layer upon layer of hidden goals. He kidnapped Marcone in order to get access to the Archive. He kidnapped the Archive in order to get Thorned Namshiel to expose himself, or did he? And why was Lucifer willing to give them a massive hellfire cheat -- what was so important in the events of that book to make it worth giving Uriel the opening to gift Harry with Soulfire?
Was the massive hellfire pentacle another test? Is it a trick Nic will be using in the future either to give Dresden a binding circle around the Last Titan, or to surround large numbers of Outsiders so they can be banished like the cornerhounds?
The one big escalation we see over the course of the books is his recruitment of more and more powerful hosts for the coins. It really looks like most hosts have been just ordinary schmucks -- like Sanya the easily-manipulated angry youth. Even the spellcasters are fairly minor -- Cassius was a sorcerer for fifteen hundred years and was the weakest sidekick in Dead Beat.
In Death Masks, Nic makes a concerted effort to recruit Harry. In Small Favor, Nic is offering coins to the Archive and Marcone. In Skin Game, he's given coins to a powerful warlock and to the genoskwa, and is trying to recruit Grey. That's some serious escalation. And there's no reason he couldn't have been giving coins to more powerful mortals and scions all along, is there?
And in keeping with new Peace Talks information... wild speculation:
With starborns appearing every 666 years, could Nicodemus be starborn? There's nothing that says a starborn has to end up a good guy (or even a guy). WAG starborn list: Nic, Rashid, Mavra, Harry+Elaine.
To ONLINE ClintACK, I really like your bit of wild speculation at the end.
I made the same observation regarding Nicodemus's potential elsewhere. As a Starborn he has dominion over Outsiders and that very fact may inform his motivations in the whole series.