if you have a network of websites it's better to use 3rd-party tracking tools.

Although there's no concrete proof, it's easy for Google to see your network of sites on a manual review just by checking your Analytics & Adsense ID's. The algorithms won't check for it (as far as I know), but a manual spam review would probably want to uncover and destroy your whole network. So, as Xela explained, try and stick to third-party wherever possible.