I see a lot of Ad Networks that are under-performing
Speaking for myself, after years of experiments and studies, I don't block anything on AdSense for my diverse portfolio.
Because unless I have access to the AdSense inner-working algorithms, it is humanly impossible for me to know what Ad/Ad-Network is under-performing to who/what/when/how.
In fact I can't even begin to quantify 'under-performance', unless
I somehow simulate being every single visitor on my sites exposed to my every served Ad/Ad-Network
I get competitive alternatives to those under-performing Ad/Ad-Networks from the non-AdSense marketplace (none exist for me at least)
What I for one think is that there is a reason why things under-perform, and mostly it has nothing to do with what I can't really control (the Ad/Ad-Network, etc) but everything to do with what I can control (to who/what/when/how Adsense is being served, ie traffic, quality etc etc.)