Killcoil isn't directly linked to tarnishing, but also doesn't really kill growth in a system. As it is, the ability for the silver ions to positively impact the water is relatively minimal due to the low mass of the coil itself. Likewise, copper works just as well and in the watercooling industry, most components are copper-based, which also ionizes into the water. In short, the copper from cooling blocks is going to ionize far more copper into a loop than the silver kill coil will and neither is enough alone to combat microbial growth.
The PTNuke debate goes back to the early 2000's when EK and a few others were having plating issues with nickel on block finishes. This has long since improved across the board, and there hasn't been as much focus on it in recent time. Would I use it? Certainly, and I have....I just don't have any more at the moment and I've been successful with some anti-microbial drops designed for aquariums. I am also supportive of EK and Alphacool clear coolants - I've used both and have had good experiences with them. I am not supportive of opaque coolants meant to provide creamy or swirl effects - these often can result in gunking, congealing or separation under some use conditions, such as very high loads, periods of non-use or certain other chemicals often found in cooling loops, like oils from radiator manufacturing.