Some (well ranking!) competitors of mine buy backlinks from a site that was penalized by panda / penguin back in 2012. This website still has good metrics (PA/DA/TF/CF ~ 35-45). But I mean, this website was penalized and has not recovered since then. I can see that because this site ranks very low for its main keywords although these keywords are low competitive. Also the pagerank dropped down from 5 to 0 in 2012 and still is 0! Obviously google does not like this site anymore! Normally I would never consider to get a backlink from there... However, competitors still buy backlinks from this site. Are they just dumb (I dont think so because they rank well!) or can backlinks from penalized websites still be powerful? Interested to hear your opinions!!