Here is what we found based on old sites we had with legacy links, older sites with spam stuff , as well as all our newer link building (read last twelve-eighteen months) that has relied on citations and a mix of PBN only (some old fashioned machined out spun articles and most original content)
Sites with spammy links got hammered BUT !! ...If they had a good mix of niche relevant links pointing to them they survived just fine
We looked at a lot of our forgotten sites - still live but unwanted by us and dormant for a long while but were still ranking well - we used a lot of tools back in the day - GSA being our favoured gun. These sites have not had links built to them for well over a year - there was a marked difference though in terms of outcome. If a site had only had thin content web 2.0s or spam pointing to it - it had a marked drop - if it had the same type of links but with a healthy dollop of niche PBN it increased its rankings. It looks like spam can be tolerated to some degree, provided the Google monster sees enough quality links to counter it.
However we have weaned ourselves of tiered link building using tools for nigh on two years and even though it may still represent a shortcut that works - don't use it for any site you care about.
Relevance really really matters...we have never considered Google stupid but they really do seem to be putting out smarter updates
Remember back in the day? A boatload of links from any kind of private / public network would rocket rankings. It was a guaranteed boost - buy 50 links and watch you rankings rise....happiness!
In this update the most profound difference we noticed with sites that sailed to the top was that they were backed not just by good quality links, but that these links were niche relevant. This matters and makes life even more difficult for link sellers. No longer can you have ten blogs and post out to ten different money sites with the same effect - there is real power in relevance - None of us know for sure what goes on inside Google Towers but they have absolutely turned a dial on this. If you really want a PBN to work for you whether private (as in your own) or public (no such thing as a private network that sells links) you best be sure the content on those sites relates to content on your money site - sure the page rank/trust flow/ domain authority etc might carry weight but a whole lot less if the on site content does not relate to your niche.
Ignore on page at your peril....you cannot sustainably throw up junk sites and expect Google love.
No more three page adsense wonders...If you are going to create a money site make it worthy. Original content - all titles in place, some media (video/image) make it look like a site someone cares about - if at all possible Silo that site at least three pages deep in a couple of categories. Google is becoming masterful at reading and understanding content/context - the other issue this throws up is that it no longer seems as reliant on incoming anchor text to your site to understand it - so be real careful with anchor text % - we are now going no more than 2% money keyword.
Create readable original content on your money sites. Sure it's more work but your site will stay in the index. There are more words written about on page that I can go into here - PM me if you want a guide.
I know a lot of people got burned in the update and back in 2012 we did when Build My Rank and others got hit. It's why we started building our own networks - we just wanted to be bullet proof or at least be as sure as we can be. SEO is not dead - as always it simply changes.
Hope this is of some help - everything here is our direct data driven experience of the last few weeks - not guesswork.