@tiiberius
In general public PBNs are very risky for at least few reasons.
But also there are some people who aren't able to build own PBN myself.
The next thing - all kind of BL crawlers - such Ahrefs, Moz, Majestic etc...
Ahrefs are pretty easy and quick to block without any footprint, as they use ovh.com
Moz also is easy to block, they currently use wowrack.com
But when it coming about the Majestic - they are use much clever crawling methods.
You need a lot of data from many traffic sites to know all their IP ranges.
In other case - only robots.txt is the option, which is the footprint.
For this reason, you can find Majestic in most of robots.txt files.
Of course, you can block them on User Agent string - but you never know if in reality it is the Majestic.
Also for example my crawlers testing User Agent for such kind of blocking.
So, the only effective and safe method is build the real site with traffic.
And use the links from such PBN via blog / other sites - ONLY in this case you haven't any footprint.
All the other tricks, such change html link to text can be detected.
If Google will detect such tricks - they will reset the links power.
Also when Google will detect some "unnatural" behavior,
such bad anchor text ratio or any other signs - there is pretty clear - what's come on.
As I am always repeating - SEO's doing that many mistakes, that even in result they don't know how Google detected them.
Bad ratio / unnatural link building schema are the most common things.
Cheers, Greg.
